Mối tương quan giữa biến đổi khí hậu và tỷ lệ giới tính trẻ sơ sinh trong giai đoạn 2008 2016

VIETNAM NATIONAL UNVERSITY
SCHOOL OF INTERDISCIPLINARY STUDIES

NGUYEN THI VAN

ASSOCIATION BETWEEN CLIMATE CHANGE AND
MALE FEMALE RATIOS OF NEWBORN IN HANOI
DURING THE PERIOD OF 2008 - 2016

GRADUATION MASTER THESIS
IN CLIMATE CHANGE

Hanoi - 2018


VIETNAM NATIONAL UNVERSITY
SCHOOL OF INTERDISCIPLINARY STUDIES

NGUYEN THI VAN

ASSOCIATION BETWEEN CLIMATE CHANGE AND
MALE FEMALE RATIOS OF NEWBORN IN HANOI
DURING THE PERIOD OF 2008 - 2016

GRADUATION MASTER THESIS
IN CLIMATE CHANGE
Major: Climate change
Code: Pilot program

Supervisor: Assoc. Prof., Dr. Mai Van Hung

Hanoi - 2018


DECLARATION
I hereby declare that the thesis entitles “association between climate change and
sex ratio of newborn in Hanoi during the period of 2008 – 2016” submitted for
the completion of the master degree in the VNU School of Interdisciplinary Studies,
Vietnam National University, Hanoi, Vietnam and the results therein are my own
personal original, research work carried out under the guidance of Assoc Prof. Dr.
Mai Van Hung and Dr. Misao Fukuda. All of the materials or data, which were in
the course of this study, have been duty acknowledged by way of bibliography. All
references have been done legally, according to regulations of the VNU SGS. The
work included in this thesis is original and has not been submitted for any other
publication.
Researcher

Nguyen Thi Van


ACKNOWLEDGEMENTS
First of all, I would like to express my sincerest thanks to my supervisor, Associate
Professor Doctor Mai Van Hung who shared his invaluable knowledge and
experiences so openly and candidly to guide and encourage me to fulfill the study. It
must to say, I am so grateful for his great patience, persistence and understanding to
follow my tough schedule during research implementation. Thank you Doctor
Misao Fukuda for inspiring me, supporting me with method and materials despite
we have never met each other before. Without them, none of this study would be
done.
I am privileged to extend my sincere gratitude to Dr. Luu Bich Ngoc who
contributed precious ideas in order to fill the gap and improve the study. Thanks to

Dr. Nguyen Trong An, MD, former Deputy Director of Department of Child Affairs
(Ministry of Labor, Invalids and Social Affairs), Mr. Nguyen Dai Duong, Head of
the Organization and Administration Department (Hanoi Population and Family
planning branch), Ms. Nguyen Thuy Linh, Hanoi Department of Health, I could
access raw data of yearly and monthly live births by gender (male and female) in
Hanoi from 2008 to 2006 after my almost 1,5-year-effort in vain. I would like to
express my special thanks to Mr. Nguyen Quang Ha, Chief of Office of National
Hydrometeorology Center and my classmate as well, provided me with the record
of temperature of Hanoi from 2007 to 2016.
My appreciation to all professors of the master programs for providing us with
informative lectures and sharing their practical experiences and Department of
Training Management and Student Affairs, especially Dr. Du Duc Thang, for the
kindness and assistance.
Last but not least, my warmest thanks must be to my amazing family, boyfriend,
friends for their love, constant encouragement so that I could complete the study.


CONTENT
LIST OF TABLES ...................................................................................................... i
LIST OF FIGURES.................................................................................................... ii
GLOSSARY ................................................................................................................3
INTRODUCTION .......................................................................................................4
1. Rationale of the study ...........................................................................................4
2. Research question .................................................................................................7
3. Patients and scope of the study.............................................................................7
4. Limitations ............................................................................................................9
5. Thesis structure ...................................................................................................11
CHAPTER 1: BACKGROUND ...............................................................................12
1.1.

Factors affecting male to female ratios of newborn .......................................12

1.2. Climate change and the secondary sex ratio ......................................................25
2.1

Global warming & climate extremes ..............................................................29

2.2.

Data Analysis ..................................................................................................33

CHAPTER 3: RESULTS AND DISCUSSION ........................................................35
3.1.

Correlation between yearly mean temperature differences (0C) and male to

female ratios of births from 2008 to 2016 .................................................................35
3.2.

Correlation between monthly mean temperature differences (0C) and male to

female ratios of births from 2008 to 2016 .................................................................39
CONCLUSION .........................................................................................................43
RECOMMENDATION ............................................................................................44


LIST OF TABLES
Table 1. Sex ratios at age 0 by province and urban or rural residence in Vietnam ..22
Table 2. Sex ratios of live births by parity in hospital data from Hanoi and Ho Chi
Minh City ..................................................................................................................23

Table 3. Monthly and yearly mean temperature differences in Hanoi from 2008 to
2009 ...........................................................................................................................31
Table 4. Monthly and yearly male to female ratios at births in Hanoi from 2008 to
2016 ...........................................................................................................................32

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LIST OF FIGURES
Figure 1. Pearson correlation coefficient (r) between yearly mean temperature
differences and male to female ratios (sex ratios) at births of Hanoi from 2008 to
2016 ...........................................................................................................................35
Figure 2. Yearly mean temperature differences (0C) and male to female ratios of
births (sex ratios at births) from 2008 to 2016 in Hanoi ...........................................36
Figure 3. Pearson correlation coefficient (r) between monthly mean temperature
differences in 2015 and sex ratios at births in 9 months later ...................................39
Figure 4. Monthly mean temperature differences (0C) in 2015 and male to female
ratios of births (sex ratios at births) from October 2015 to September 2016, 9
months after the previous temperature assessment ...................................................41

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GLOSSARY
The sex ratio at birth refers to the number of boys born alive per 100 girls born
alive (Handbook of Vital Statistics Systems and Methods, 1991). In other words, it
is male female ratio of newborn. In addition, the secondary sex ratio is defined
as the ratio of male-to-female live births (United Nations DoEaSA, 2011).
Therefore, in the present study, these three phrases were used alternatively together.
Climate change is a change in the state of the climate that can be identified

by changes in the mean and/or the variability of its properties and that persists for
an extended period, typically decades or longer.
Global warming is a phenomenon believed to occur as a result of the build—up of
carbon dioxide and other greenhouse gases. It has been identified by many scientists
as a major global environmental threat (Glossary of Environment Statistics, 1997).
Mean temperature difference or temperature anomaly means a departure from a
reference value or long-term average. A positive anomaly indicates that the
observed temperature was warmer than the reference value, while a negative
anomaly indicates that the observed temperature was cooler than the reference
value. It is calculated by the sum of monthly mean temperature minus the mean
temperature during the previous 9 years for the same month/12 (Fukuda at el.,
2014).

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INTRODUCTION

1.

Rationale of the study
a. Climate change- a widespread phenomenon has massive negative
impacts on human beings

―Summers to get hotter,‖ ―Climate change ―kills‖ 4000 people and ―digests‖ 1200
billion dollars,‖ ―Global Warming Seen as Security Threat,‖ ―Global warming a
bigger threat to poor,‖ ―Tibet’s glacier’s heading for meltdown,‖ ―Climate change
affects deep sea life,‖ ―More than half a million could die as climate change impacts
diet – report‖, ―February breaks global temperature records by 'shocking' amount.‖
These are just a few of the many headlines that crossed the wires in 2016, and they

have elicited widespread concern. Scientific evidence for warming of the climate
system is unequivocal - Intergovernmental Panel on Climate Change (IPCC). The
Earth’s average surface temperature has risen by 0.76° C since 1850 (IPCC, 2007).
In its Fourth Assessment Report projects that, the global average surface
temperature is likely to rise by a further 1.8 - 4.0°C this century, and by up to 6.4°C
in the worst case scenario. Even the lower end of this range would take the
temperature increase since pre-industrial times above 2°C – the threshold beyond
which irreversible and possibly catastrophic changes become far more likely.
The climate change can affect every person and our health directly through
increases in temperature. Although it is undeniable that global warming may bring
some localized benefits, such as fewer winter deaths in temperate climates and
increased food production in certain areas, the overall health effects of a changing
climate are likely to be overwhelmingly negative (World Health Organization,
2015). Between 2000-2100, the heat related deaths will rise by 150,000
(Environmental Protection Agency, 2004). Such increases may lead to more
extreme heat waves during the summer while producing less extreme cold spells

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during the winter. Particular segments of the population such as those with heart
problems, asthma, the elderly, and the very young can be especially vulnerable to
extreme heat. There can be extreme floods and droughts, hurricanes. In 1877, a
strong El Nino event caused failure of the monsoon rains in south and central India
which resulted in the intense famine of 6 to 10 million deaths due to the drought
(World Health Organization, 2003). The El Nino effect on disasters is strong
enough to be apparent at the global level (Bouma, M.J. et al., 1997).
In addition to natural disasters, climate change can have devastating effects on
human societies with disease outbreaks (World Health Organization, 2003).
Increase in temperature reduce the time taken for vector populations to breed and

the incubation period of the pathogen (e.g. malaria parasite, dengue or yellow fever
virus) meaning that vectors become infectious more quickly (ISSN, 2013). Over 2.5
billion people are at risk, and there are estimated to be 0.5 billion cases and more
than 1 million deaths from malaria per year (World Health Organization, 1998). In
1996, Currents of change: El Nino’s impact on climate and society, Lien and Ninh
found that the number of dengue cased increased in El Nino years. In July 1995 a
heat wave in Chicago, USA, caused 514 heat-related deaths (12 per 100 000
population) and 3300 excess emergency admission (Whitman et al., 1997). The
morgues were full and bodies had to be stored in refrigerated trucks. From 10 to 20
July, daily temperatures ranged from 34-40oC, with the highest temperature on 13
July. The maximum number of deaths occurred on 15 July (Dematte J.E. et al.,
1998).
Following the Chicago heat wave in 1995, Semenza et al. conducted an important
study on victims who died during the climate extreme and controls who lived near
the case, matched for age and neighborhood. Individual risk factors for dying in the
heat wave were identified: chronic illness; confined to bed; unable to take for
themselves; isolated; without air conditioners. A comparison of mortality rates in
three heat waves in 1966 by age group, sex and ethnic group indicated that women
and white people were at more risk (Bridger and Helfand, 1968).

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b.

Male female ratio of newborns in the world reflects a downward trend

probably due to environmental exposures, but has no assessment in Vietnam.
The ratio between male and female newborns was indicated to be out of the
constant norm around the world with a small surplus of boys. There’s a subtle

change in the secondary sex ratio have been reported during last century from a
number of countries with contradictory results (Fukuda et al., 2014). Countries from
which studies have shown a reduced sex ratio at birth include: Denmark (Moller,
1996), England and Wales (Dickinson and Parker, 1996), Canada (Allan et al,
1997), the Netherlands (Van der Pal-de Bruin et al, 1997), Germany (Bromen and
Jockey, 1997), U.S.A (Davis et al, 1998), Sweden, Norway and Finland (Moller,
1998) and Japan ( Parazzini et al, 199858; Mizuno 2000; Davis et al, 200714;
Fukuda et al, 2014). On the other hand, there were countries reporting a stable trend
in the offspring sex ratio including: Australia (Parazzini et al., 1998) and Argentina
(Parazzini et al., 1998). Those showing increasing trends: Italy (Parazzini et al.,
1998), Spain (Parazzini et al., 1998), Australia (Parazzini et al., 1998) and South
American (Grech, 2013). Parazzini et al. reported that among 29 countries from the
World Health Organization database the proportion of males decreased in 16,
increased in 6 and remained stable in 7. The reason for the differences among these
29 countries is unidentified but may represent local causes. Overall, more than half
of the countries evaluated showed decreasing trends in the ratio of male to female
live births. After the Seveso accident with exposure to dioxin, there was an excess
of females born in the period April 1977 to December 1984 (Mocarelli et al., 1996).
This ratio declined in the years from 1985 to 1994 and was thereafter no longer
significant. There have also been reports of declining proportion of male newborns
during the last century from the Netherlands (Van der Pal-de Bruin et al., 1997),
England and Wales (Dickinson and Parker, 1996), as well as from Canada (Allan et
al., 1997). This decline in the sex ratio of newborn remains unexplained, but has
been speculated to be related to exposure to xeno-oestrogenic chemicals. Moller H.
in 1996 reported a decline in the proportion of male newborn babies in Denmark

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between 1951 and 1995, possibly indicative of new reproductive hazards ascribed to

environmental oestrogens.
If this hypothesis holds true, a similar decline in other countries like Vietnam with
such environmental exposures would be expected. Therefore, the research is
conducted with a hope that it indicates possible relationship between male:female
ratio of newborn and environment factors related to climate change, especially
global warming in Hanoi, Vietnam.
The study is aimed to execute a basic research into the possible correlation between
temperature warming and sex ratio at birth in Hanoi, which may inspire and support
more comprehensive and intensive researches in the future. It uses worldwide
methodology of temperature related birth sex ratio variation that have never been
tested in the context of Vietnam before. Therefore, the finding can adds to the
growing body of literature in human beings suggesting a role of environmental
temperature in the secondary sex ratio variation. Otherwise, the paper is expected to
mark an important step forward towards development of interdisciplinary research
in climate change and reproductive health in Vietnam.
In addition, the possible result that sex ratio of birth is influenced by climate change
can reinforce message of protect environment from human-caused climate change
to be more convincing and powerful.
2.

Research question

In order to achieve the mentioned aims of this study, the research paper is to deal
with the question: ―Is there any possible association between climate change and
male: female ratio of newborn in Hanoi?‖
3.

Patients and scope of the study

Global warming. Global warming refers only to the Earth’s rising surface

temperature, while climate change includes warming and the ―side effects‖ of

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warming - like melting glaciers, higher seas, heavier rainstorms, or more frequent
drought. Said another way, global warming is the most obvious symptom of the
much larger problem of climate change (Caitlyn Kennedy, C., Lindsey R., 2015).
All signals of global warming can have a range of impacts on physical, mental, and
community health. However, in this study, link of male:female ratio of birth and
climate change is assessed through impact of global warming on sex ratio of
newborn.
Hanoi. Vietnam is considered as one of the countries to be severely affected by
climate change (IPCC, 2001). Vietnam’s temperature and precipitation trends have
been greatly different among regions during the last 50 years. Annual average
temperature has increased by 0.5°C nationwide and annual precipitation has
decreased in the North and increased in the South (MONRE 2012 a, b, 2013). Hanoi
is reported by one of the top-10 largest threatened populations by climate change
including Shanghai, Hong Kong, Calcutta, Mumbai, Dhaka and Jakarta (Strauss B.
et al., 2015). Hanoi, the capital of Vietnam, comprises 29 districts, 10 of which are
urban and 19 rural. As of 2012, Hanoi’s population was estimated at 7 million, of
which 2.6 million (41%) lived in urban districts (Statistical yearbook, 2009). From
2010 to 2016, temperatures in Hanoi exceeded 39°C in summer; For instance, the
temperature record of Hanoi Lang weather station showed that it was 40.4°C and
40.8°C in June, 2010 and July 2015 respectively. Such communication is
increasingly a priority for Hanoi City, because in recent years, climate change has
affected the city with extreme consequences (National target program to response
climate change, 2007). Nonetheless, to the best of my knowledge, almost no
research has been conducted in Vietnam about association between climate change
and human reproduction health. Therefore, the aim of this study was to explore

Hanoi's perceptions of climate change and its effect on sex ratio of birth during the
last 9 years, 2008 - 2016.
2008-2016. I supposed to conduct observation and assessment of the probable
relationship between Global warming and the sex ratio of newborn in Hanoi for a

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longer period. However, Ha Tay, a former province of Vietnam, was merged as a
part of Hanoi on August 1, 2008. As a result, many governmental agencies of Ha
Tay broke up and was blended relevantly and properly with Hanoi’s ones. For
instance, Ha Tay Population and Family planning branch was blended with Hanoi
branch. “However, data synthesis among two branches met a lot of difficulties and
restraint due to the difference in methodology, collection tools. We just synthetized
some main data.‖ Said the representative of Organization and Administration
Department of Hanoi Population and Family planning branch. Hence, in order to
raise reliability of the findings and reduce pressure to collect data, the study time
should be scoped after Hanoi’s united that the study. Based on availability and
advice of data-holder, the study will test the correlation of climate change and sex
ratio at birth in Hanoi during the period of 2008 – 2016.
4.

Limitations

Sample size – As mentioned, the observation time is constraint due to Hanoi
expansion. Therefore, the number of the units of analysis I used in this study is
pretty small, 9 years, compared to 34 years of the similar research by Fukuda et al.
in Japan. As a result, it was likely for the data analysis to gain outcomes without
expected statistical significance (α = 0.05). In order to make use of all contributions
deriving from numerical findings, in some cases, the statistic significant value was

more flexible (α = 0.1).
Lack of prior research studies on the topic – citing prior research studies forms
the basis of the literature review and helps lay a foundation for understanding the
research problem are going to be investigated. However, in Vietnam, there was no
similar study that prevented me from discovering a limitation which can serve as an
important opportunity to identify new gaps and fix them in my study. Although
there were some former researches in the world, I need apply gingerly when putting
into context of Vietnam; unless, an entirely new research typology would be
required.

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Longitudinal effects – Normally, most researchers of correlation between
temperature and sex ratios of newborns are working in medical or biological fields
(*)

. They have profound knowledge and experiences to capture and explore

underlying mechanism of possible correlation between two above variables. With
my shortage of knowledge and experience about this topic, in my very first study, I
expected to find out just whether there is association between climate change and
sex ratios at births or not. In order to investigate a precise mechanism of the
association, for example, how global warming influences fertilization of XY
embryos/ XX embryos, a clinical research with experimental demonstrations is
encouraged in the future.
Cofounders – Before temperature-related birth sex ratio bias was proved in some
parts of the world, many studies found out the association between offspring sex
ratio and a succession of external factors like cigarette smoking, air pollution or son
preference. However, due to insufficient database or theoretical basis of those

factors’ impact on sex ratio in Vietnamese context, we couldn’t eliminate them in
the research model. Hence, within the study, we supposed these factors to hold the
unchange, stable influence on the sex ratio at birth over the targeted period.

(*)



Climate change, Tohoku earthquake and sex ratio of fetal death & birth: The male to female ratio of
newborn in Japan in relation to climate change, Tohoku earthquake and fetal deaths in 2015 by
M.D., Ph.D. Misao Fukuda, M&K Health Institute, 30-9 Kariya, Ako, Hyogo 678-0239, Japan.



Sex Ratios at Birth and Environmental Temperatures in 1999 by Prof. Dr. Alexander Lerchl,
Institute of Reproductive Medicine, University of Münster, Germany.



Temperature-related birth sex ratio bias in historical Sami: warm years bring more sons in 2008 by
Biologist Samuli Helle, Section of Ecology, Department of Biology, University of Turku, 20014
Turku, Finland

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5.

Thesis structure

This thesis includes a general introduction, followed by three main chapters, and
conclusion. In the introduction section, I have outlined my thesis’s aims, research
questions, expected contribution and scope of the study, accompanied by the
limitations which cause the thesis’s imperfection. Chapter 1 – background, will
proceed with an overview of study area involved in sex ratio at births, climate
warning and relevant literature of relationship between temperature and
male:female ratios of newborns. In chapter 2, detailed descriptions of the field and
analysis method used to complete this study are presented. Chapter 3 summarizes
the results and illustrates the data in graphic form. Discussion imported from the
results is presented in this chapter as well. Following by, the conclusion section is a
brief summary of the major findings about correlation between climate change and
sex ratios at births in Hanoi from 2008 to 2016. Finally, follow-up activities and
suggestion are offered in the recommendation.

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CHAPTER 1: BACKGROUND
1.1.

Factors affecting male to female ratios of newborn

There is a number of factors associated with changing sex ratio of newborn. In
1973, Trivers and Willard formulated a hypothesis to explain these fluctuations:
―Theory and data suggest that a male in good condition at the end of the period of
parental investment is expected to outreproduce a sister in similar condition, while
she is expected to out reproduce him if both are in poor condition. Accordingly,
natural selection should favor parental ability to adjust the sex ratio of offspring
produced according to parental ability to invest. Data from mammals support the
model: As maternal condition declines, the adult female tends to produce a lower

ratio of males to females. ... Parents in better condition would be expected to show
a bias toward male offspring.‖ In climate change, Tohoku earthquake and sex ratio
of fetal death & birth by Fukuda et al. in 2015, sixteen factors were involved in the
influence on the sex ratio of births, including cigarette smoking, dioxin, methylmercury, air pollution, nuclear radiation, electro-magnetic fields, cervical mucous,
coital frequency, ovulation patterns, Mothers’ age at menarche, parental age,
diet/nutrition, pregnancy related disorders, psychological stress, economic status,
war and terrorist attacks. Besides, there were some researches which reported Asia
had the highest proportion of males in the world due to its strong preference for
sons (Guilmoto, 2007; Belanger et al., 2003). Recently, with amazing availablity
and accessibility of abortion service, with one or two-child policy, the influence of
male chauvinism is visible in some Asian countries, including Vietnam. In addition,
natural disasters have been reported to associated with offspring sex ratio such
floods (Lyster, 1974), hurricans (Grech and Scherb, 2015), droughts (Flatø and
Kotsadam, 2015), earthquakes (Saadat, 2008; Torche and Kleinhaus, 2012;
D’Alfonso, 2012; Doger et al., 2013). Of interest, temperature was involved in
variation of sex ratios at birth. A higher proportion of boys was found at higher air
temperature (Lerchl A., 1999; Grech et al., 2000) while in another cencus in 2002,

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Grech and his team reported more boys were born in northern with colder latitudes
compared to the southern ones in North American.
a. Cigarette smoking
The impacts of cigarette smoking on reproduction health have been raised for many
years with a number of researches. In 1912, Tidswell were the pioneer for the
connection between male to female ratio of newborns and paternal smokers. He
found that the ratio was higher in paternal non-smokers than paternal smokers. After
almost a century, Fukuda et al. in 2002 reported the similar result when he assessed
11815 singleton newborn delivered by 5372 women in Japan. The lowest sex ratio

among offspring was found when both parents smoked in excess of over 20
cigarettes per day (Fukuda et al., 2002).
However, in Sweden and Denmark, two studies by Cnattingius et al. And Obel et al.
respectively were unable to find effects of paternal and maternal smoking on the
offspring sex ratio. The first study analyzed link of 1,552,790 births and maternal
smoking habits in Sweden from 1983 to 1997. It considered those who burn more
than 10 cigarettes per day as heavy smokers. Ten cigarettes per day is relatively
low, compared to others studies, most define heavy smokers with consumption of at
least 20 cigarettes per day (Mills et al., 2003). The other study by Obel et al. which
failed to find effects of parental smoking on newborn was believed that only
included a relatively small number of heavy smokers (Fukuda et al., 2003).
In contrast, a recent analysis and community survey of births in Liverpool
investigated a low sex ratio of newborn among parents who were both heavy
smokers with evidence for a dose-response (Fukuda et al., 2015). Besides, Viloria et
al. in 2005 evaluated a possible mechanism behind a potential skewed sex ratio in
heavy smokers. They reported there’s an enrichment of X spermatozoa after swimup in males with high tobacco consumption suggesting impairment of the motility
of Y-bearing spermatozoa (Fukuda et al., 2015).

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In brief, there are some researched suggest that smoking, especially heavy smokers,
does affect negatively the male:female ratio of newborn. Studies which fail to
confirm this statement tend to have included too few heavy smokers. However, a
number of confounding factors or co-factors may influence the secondary sex ratio
and additional studies are warranted to substantiate the effect of smoking on the
ratio of male to female live births (Fukuda et al., 2015).
b. Dioxin
Dioxins are believed to have a devastating effect on the success of human
reproduction. The studies of Mocarelli et al. both in 1996 and in 2000 reported

paternal exposure to TCDD (Tetrachlorodibenzodioxin) have association with a
lower proportion of male births. After Sevesco industrial accident in the year of
1976, there was a distinct reduction in sex ratio at birth 9 months when both parents
were living in the most contaminated zone (Mocarelli et al., 1996).
Ishihara et al. (2010) report that mice prenatally exposed to TCDD had a decreased sex
ratio fertilization and not during the spermatozoa stage owing to reduced XY 2 cell
embryos despite no change of Y-bearing/X-bearing sperm ratio (Fukuda et al., 2015).
c. Methyl-mercury
In 2016, Yorifuji and Kashima with a study of methyl-mercury contamination in
Japan found a significant decreased secondary sex ratio associated with methylmercury exposure. Further results from this study including 20,487 births suggested
a more pronounced effect on male fetuses (Sakamoto et al., 2001). ―However, the
Japanese study was ecological, with no direct measures of methyl-mercury and,
thus, the findings could be affected by measurement error and by paternal rather
than maternal exposure. Other evidence has not reported any clear association with
methyl-mercury exposure‖ (Timmermann et al., 2017).

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d. Air pollution
While Lloyd et al. found a considerably higher sex ratio of newborn during the
years 1964 – 1970 in central Scotland, William et al. declared low sex ratios of
newborn children in polluted areas. In addition, Lichtenfels et al. showed in 2007
that there was a negative correlation between air pollution and sex ratios in human
with similar results from a mouse model in Sao Paulo, Brazil. However, Williams et
al. in 1995 reported that at times of low intensity of air pollution the sex ratio were
not consistently affected and the sensitivity was determined to be too low to be
reliable screening measure for detecting health hazards from industrial air pollution.
e. Nuclear radiation
In 2004, Peterka with his coworker assessed the impacts of Chernobyl nuclear

accident which caused a disaster of radioactive release and pollution on April 26th
in 1986 in Ukraine. They found a decrease in the sex ratio of newborns in the Czech
Republic near Chernobyl, in seven months after the accident, in November 1986.
This finding suggested a selective negative effect of irradiation on male foetuses
during the second of third month of development. In addition, a negative correlation
between proportion of male births and radioactive exposure was found (Peterka et
al., 2007). In contrast, there was a raising sex ratio between 1986 and 1991 in an
area in Bavaria exposed to oradioactivity, in the former GDR, and in West Berlin
(Scherb and Voigt, 2007). Furthermore, in 2007, Mudie et al. found no sigificant
association between male to female ratio of newborns and parental radiation
exposure when they conducted a study of nuclear testing in Kazakhstan.
f. Electro-magnetic fields
A cross-sectional study among miitary men was performed in the Royal Norwegian
Navy by Baste et al. in 2008. It concludes information about work and workplaces
close to equipment emitting radiofrequency electromagnetic fields. In the results, it

15


was reported that a lower sex ratio at births among higher exposed employees of
10,497 respondents in the survey.
Irgens et al (1997) suggested that workplaces of parents impact on the frequency of
mael offprings when they found the sex ratios of births was significantly reduced
for people, espescially women, employed in industries, working with aluminium,
iron and manganese leading to increased electro-magnetic fields.
g. Cervical mucous
It have been hypothesized that Y-bearing spermatozoa swims faster than the Xbearing spermatozoa (Fukuda et al., 2015). In cervical mucus Y-spermatozoa exerts
an increased penetrability in the 1-2 day period prior to ovulation, followed by a
decrease in penetrability following ovulation (Martin, 1997). It is proposed that Y
sperms gain more advantage at ovulation rather than following ovulation.

Therefore, the probability of a male conception would be high for inseminations
early in the fertile period, increase for inseminations closer to ovulation and decline
for insemination thereafter. Whether this hypothesis is correct or not is still
controversal and big studies have not yet been conducted.
h. Coital frequency
The early months marriage are often characterised by a high coital frequency and
high sex ratio of births (Fukuda et al., 2015). In Australia the sex ratio for
conceptions fell dramatically from 1.094 in the first month of marriage by 1.060 in
the following months (Renkonen, 1970). In seven East African groups the lower sex
ratios of births in non-co-resident polygynous (0.873) compared to monogamous
(1.141) women is caused of the assumend lower coital frequency in women who
must share their husbands with co-wives resident elsewhere (Whiting, 1993;
Borgerhoff Mulder, 1994).

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i. Ovulation patterns
In 1995 Weinberg et al. was the first one to report that a mean follocular phase
length of 15.4 days in 69 women produced more males than females and a mean
follicular phase of 17.6 days in 64 cycles produced more females. Furthermore,
Helle (2008) supported the follicular hyphothesis by Weinberg er al. when reporting
that women with long menstrual cycles tend to have more daughters. However, a
large scale studies are currently avaible to subtaintiate a possible association
between follicular phase lenth and the sex ratio at birth (Fukuda et al., 2015). Gray
et al. (1998) found no consistant pattern in the sex ratio at birth associated with the
length of the follicular phase in a large cohort of 947 singleton live-born infants.
j. Mothers’ age at menarche
Fukuda et al. in 2011 tested 21,208 newborn delivered by 10,847 women and
suggest that a woman’s menarcheal age may be associated to her possibility of

having either a son or a daughter. As a result, they found that women entering
menarche outside the normal range, especially those with earlier menarche, would
be more likely to have female offspring partially becuase of increased spontaneous
abortion of male zygotes. However, a large Danish cohort of 177,599 liveborn
infants (all singletons) by Geller et al. in 2011 found no relationship between the
age at menarche and the sex ratio of the offspring. The study showed the lowest sex
ratio among all age groups from 9 to 18 years. The disagreement beween the Danish
study and the Japanese one is not known but may include multi-factorial differenes
such as environmental factors, demographphical factors, ethinical factors or other
unknown factors, which may also affect age at menarche and offspring sex ratio
(Fukuda et al., 2011).
k. Parental age
Juntunen et al. in 1997 reported that 35-year-old mothers or older mothers had 7%
more female than male newborn compared to younger mothers. The sex ratio of

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births in Denmark from 1980-1993 increased to 1.066 when the fathers aged 13-24
years and decreased to 1.041 when the father aged around 35 (Jacabsen et al.,
1999). The male birth proportion in U.S. from 1964 to 1998 declined with
increasing age of either parents (Nicolich et al., 2000). Moreover, Matsuo et al.
(2009) reported that increasing ages of both parents was inversely related to the sex
ratio of newboen infants. However, these findings could not be confirmed by EinMor et al. in 2010.
l. Diet/nutrition
Cagnacii et al. in 2004 tested 10,239 births in a community hospital in Italy from
1997 – 2001 and reported that a low pre-pregnancy weight and a greater gain during
pregnancy were both associated with a decreased sex ratio at birth. Furthermore, the
finding that high preconceptional energy intake was related to increased male to
female ratio of births was confirmed by Mathews et al. in 2008 with a cohort study

of 740 women in UK who completes questionaires on dietary habits. They
concluded that dietary changes in a Western population could explain the overall
declining trend of sex ratio at birth.
Based on the observed male-skewed sruvival rates when bovine blastocysts were
exposed to heightened levels of glucose, a high level of circulating glucose in the
mother’s bloodstream may favour the survival of male blastocysts (Larson et al.,
2001). As blood glucose levels are highly correlated with food intake, blood glucose
level may play a small role in maternal condition. In connection with early male
embryo demise around implantation stage this finding is of interest (Fukuda et al.,
2015).
m. Pregnancy related disorders
Askling et al. in 1999 found that hyperemesis gravidarum is associated with a sex
ratio of newborn: 44.3% males when hyperemesis occurs in the first trimester,
versus 51.4% males in the Swedish population in 1987 – 1995. Moreover,

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hyperemesis gravidarum is related to raised concentration of human chorionic
gonadotropin (hCG) (Askling et al., 1999). James in 2000 supported the hypothesis
that higher hCG is associated with a lower male to female ratio at births.Other
studies aslso showed that women with hyperemesis gravidarum were more likely to
have a female child than those without hyperemesis gravidarum (del Mar MeleroMontes, 2001; Rashid et al., 2012).
n. Psychological stress
Lyster in 1974 was the first to suggest a possible link between decreased sex ratio
of newborn and stress when the investigator reported altered sex ratios after the
1952 London smog and 1965 flood in Brisbane. Furthermore, a hypothesis that
severe life events may reduce the sex ratio at birth was raised by Hansen et al. in
1999. The investigators conducted a exposed cohort consisted of 3,072 singleton
pregnancies exposed to severe life events and a control cohort of 20,337 singleton

pregnancies randomly selected among all pregnancies without such exposure in the
observation period. Consequently, the proportion of boys was 49.0% in the exposed
group and 51.2% the control group. A study by Obel et al. using General Health
Questionaire (GHQ) suggested that not only severe stress, but also moderate and
common levels of spychological distress, may decrease the sex ratio of newborns.
However, with a population-based cohort study of 1.35 million singlenton live
births in Denmark between 1980 and 2002 Khashan et al. (2009) did not find strong
evidence that prenatal exposure to severe life events was resposible for a decline in
the sex ratio of the offspring.
o. Economic status
Teitelbaum and Mantel (1971) suggested that there was a significantly positive
correltion between the sex ratio at birth and socio-economic status. In 2003,
Catalano has reported a decline in the sex ratio at birth in connection with economic
crisis stressing the German in 1991 after the unification of East and West Germany.

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In contrast, some studies ( Friedlander and Benmoshe ,1986; Maconnochie and
Roman, 1997; Marleau and Saucier, 2000; Zadzinska et al., 2008) showed that
economic background did not affect sex ratio at birth. The disagreement may be due
to variations in measurement of economic status and/or different statistical methods.
p. War and terrorist attacks
During long-term wars such as World War 1 and 2, sex ratios at birth in belligerent
countries rose and started to decline after 1 year since the wars stoped (Jame, 2014).
Zorn et al. in 2002 reported that serious spychological stress in relation to a short
war (26 June – 7 July, 1991) in Slovenia resulted in a decrease in the observed sex
ratio at birth six to nine months later.
Catalano et al. in 2005 reported three months after the terrorist attacks of September
11, 2001 sex ratios at births reduced in California and New York City. The decline

in sex ratio at birhts three months after the terrorist atttacks was due to increased
male fetal deaths (Bruckner, 2010).
q. Preference for sons
Parents’ need for sons is common in countries in East Asia through South Asia, to
the Middle East and North Africa (Hesketh et al., 2011). Sons are preferred because
they have a higher wage-earning capacity (especially in agrarian economies), they
continue the family line and they usually take responsibility for care of parents in
illness and old age (Hesketh and Xing, 2006). As a result, since 1980s, ultrasound
technology started to become available for diagnostic purposes in many Asian
countries, and the opportunity to use the new technology for prenatal sex selection
was soon exploited.
In countries where there is a combination of son preference, a small-family culture
and easy access to sex-selective technologies, very serious and unprecedented sexratio imbalances have emerged. The sex ratio at birth is remarkably consistent in
human populations at around 105 male births to every 100 female births; However,

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