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PRENATAL DIAGNOSIS

Prenat Diagn 2011; 31: 560565.Published online 27 March 2011 in Wiley Online Library

(wileyonlinelibrary.com) DOI: 10.1002/pd.2747

Distortions of sex ratios at birth in the United States;evidence for prenatal gender selection

James F. X. Egan1*, Winston A. Campbell1, Audrey Chapman2, Alireza A. Shamshirsaz1,Padmalatha Gurram1 and Peter A. Benn3

1Department of Obstetrics and Gynecology, University of Connecticut Health Center, Farmington, CT 06030, USA2Department of Community Medicine and Health Care, University of Connecticut Health Center, Farmington, CT 06030, USA3Department of Genetics and Developmental Biology, Division of Human Genetics, University of Connecticut Health Center,Farmington, CT 06030, USA

Objective The normal male to female livebirth sex ratio ranges from 1.03 to 1.07. Higher ratios in China,India and Korea reflect prenatal sex selection. We reviewed sex ratios for US births to investigate potentialprenatal sex selection.

Methods We reviewed all US livebirths from 1975 to 2002 using National Center for Health Statistics birthcertificates in 4-year intervals. We compared the sex ratios of Blacks, Chinese, Filipinos, Asian Indians and

Koreans relative to Whites. We also compared the sex ratios by birth order for first, second and third and morebirths (third+) from 1991 to 2002.

Results The male to female sex ratio from 1975 to 2002 was 1.053 for Whites, 1.030 (p < 0.01) for Blacks,1.074 (p < 0.01) for Chinese and 1.073 (p < 0.01) for Filipinos. From 1991 to 2002, the sex ratio increasedfrom 1.071 to 1.086 for Chinese, 1.060 to 1.074 for Filipinos, 1.043 to 1.087 for Asian Indians and 1.069to 1.088 for Koreans. The highest sex ratios were seen for third+ births to Asian Indians (1.126), Chinese(1.111) and Koreans (1.109).

Conclusion The male to female livebirth sex ratio in the United States exceeded expected biological variationfor third+ births to Chinese, Asian Indians and Koreans strongly suggesting prenatal sex selection. Copyright 2011 John Wiley & Sons, Ltd.

KEY WORDS: male sex selection; prenatal diagnosis; ultrasound

INTRODUCTION

Male sex selection at birth has been well-documented inChina, India, Korea and some other countries (Heskethand Xing, 2006). The cultural basis for this in Chinaand Korea is rooted in the tenets of Confucianism,which mandate a strict patrilineal inheritance (Chungand Das Gupta, 2007; Das Gupta, 2009) Sons werealso traditionally responsible for the care of eldersin the family and daughters were effectively lost totheir parents after they married. This social structuremade producing and raising male children the most

important role for women in the family. The explanationfor India may be more complex but probably alsoreflects similar patrilineal values (Das Gupta, 1987)Although these cultural preferences for male childrenexisted for centuries, it was not until the 1980s thatthe technology for prenatal sex selection, i.e. secondtrimester ultrasound to determine fetal sex and thereby

*Correspondence to: James F. X. Egan, Department of Obstetricsand Gynecology, University of Connecticut Health Center, 263Farmington Avenue, Farmington, CT 06030, USA.E-mail: jegan@uchc.edu An abstract with some of the data contained in this article waspresented as a poster at the 31st Annual Meeting of the Society

of MaternalFetal Medicine, February 11, 2011 in San Francisco,CA, USA.

provide the choice for the termination of a female fetus,was widely available.

Although contemporary laws in these countries havemade discrimination against women illegal, there is stillevidence for continuing prenatal sex selection in areas ofChina and India (Park and Cho, 1995; George, 2006; Zhuet al., 2009). A distortion in the sex ratio for a countryor a population has many social and ethical implications.The potential consequences of a surplus of males includefewer women to marry, long-term economic stressesassociated with declining population numbers, moremental health problems, increased mobility and violencein young men devoid of family responsibilities and agrowing sex industry with coercion and trafficking ofwomen.(Hesketh et al., 2005; Hesketh and Xing, 2006)

The sex ratio is defined as the ratio of male birthsto female births. The sex ratio at birth ranges from1.03 to 1.07 in most western industrialized countrieswith a median of 1.059 (Parazzini et al., 1998; UnitedNations, Department of Economic and Social Affairs,2008). Sex ratios generally decline with increasingparity and increasing age (Mathews and Hamilton,2005). A reversal of this normal trend in the sex ratiowith increasing parity might be indicative of prenatalsex selection because it may be motivated by parentswishing to be assured that there is a male heir.

Mathews and Hamilton(2005) analyzed trends in thesex ratios for US births from 1910 to 2002. They noted

Copyright 2011 John Wiley & Sons, Ltd. Received: 3 February 2011Revised: 23 February 2011

Accepted: 24 February 2011Published online: 27 March 2011

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DISTORTIONS OF SEX RATIOS AT BIRTH IN THE UNITED STATES 561

a high sex ratio for births to Chinese mothers (1.074)and Filipino mothers (1.072) but did not separatelyanalyze other Asian minorities where gender selectionis common or consider patterns associated with birthorder in these populations. We reanalyzed the sex ratiosin US births from 1975 to 2002 for various populations

and sex ratios by birth order to determine if there werepatterns consistent with prenatal sex selection.

METHODS

Various definitions of the sex ratio at birth are available(male to female, males per 100 females, males per 1000females and female to male). We defined the sex ratioas the number of male births divided by the number offemale births (Davis et al., 1998).

Using data from the US National Center for Health

Statistics (NCHS) from 1975 to 2002, we recordedthe birth sex ratios by maternal race and nationalityand birth order and grouped them into 4-year intervals(Centers for Disease Control and Prevention, NationalCenter for Health Statistics, 1975 1990; Centers forDisease Control and Prevention, National Center forHealth Statistics, 1990 2006). We included singletonand multiple births and also tracked maternal placeof birth. Maternal race and nationality was based onbirth certificate data and may include individuals ofmixed race. Through 2002, NCHS classified mothersby a discrete race and/or nationality category, i.e. therewas no overlap in the groups reported. For example,White excludes all Asian and Pacific Islanders. Some

race/nationality classifications changed during the study,so certain categories were only available for limitedtime periods. We confined our analysis to those Asianpopulations residing in the United States with the highestnumbers of births. We assumed that the reporting ofgender at birth was equally accurate for all populationsand all time intervals included in this study.

Ratios for Black, Chinese, Filipino, Asian Indian andKorean were compared to those reported for Whitebirths. We also determined sex ratios by birth orderfor first, second and third or more children (third+),the mothers place of birth (i.e. in the 50 UnitedStates and the District of Columbia, or elsewhere) andsingleton and multiple births from 1991 to 2002 by raceand nationality. The quadrennial data for 1975 2002allowed the analysis of race/nationality for many groups,but 19912002 was the only time period where data forparity was available for these specific Asian and PacificIsland populations in the United States.

Because the data is an entire population, statisticalsampling errors are not present and the results can beinterpreted directly. However, we also used the statisticaltest of proportions suggested by Mathews and Hamilton(2005) where the data is considered to be one possibleset of outcomes that could have arisen in similar cir-cumstances. This provided a measure of the strength ofthe observed patterns given the size of the populations.

For the large number of comparisons involving differ-ences in the sex ratio for populations relative to White,

Figure 1Male to female sex ratios at birth in the United States in4-year intervals from 1975 to 2002 by selected races

we used a two-tailed test with p < 0.01 considered sig-

nificant. For the more limited analyses that evaluatedwhether there was an excess of males with higher par-ity, we compared the ratios using a one-tailed test with

p < 0.05 considered significant.This data is publicly available, de-identified data so

our institutional human subject review board approvalwas not required.

RESULTS

The male/female sex ratio for all 105 939 732 US births

from 1975 to 2002 was 1.050. The ratio declined from1.053 in 19751978 to 1.048 in 19951998 beforereturning to 1.053 in 19992002. For White births,the ratio declined from 1.058 in 1975 1978 to 1.050in 19992002 (Figure 1). At the same time, the ratiofor Black births increased from 1.028 in 19751978 to1.032 in 19992002 and this was significantly differentfrom White births for all quadrennials (Table 1). Thehighest ratios were seen in several Asian-Americanpopulations. Ratios exceeded 1.08 for Filipinos from1983 to 1990 and Chinese from 1995 to 1998. Chinesehad significantly higher sex ratios when compared toWhite births for quadrennial intervals from 1991 to 2002but there was no significant difference for 19751990.Filipino populations also showed significantly elevatedratios for the time periods of 19831990 and 1995 2002(Table 1).

From 1991 to 2002, NCHS provided additionalnationality sub-categories allowing the analyses toinclude Asian Indians and Koreans (Figure 2). Through-out this time period, both Asian Indian and Korean birthsshowed higher sex ratios relative to Whites, but thesedifferences only reached statistical significance for the19911994 quadrennial (Table 1).

For 1991 2002, information was also available forsex ratios by birth order. Figure 3 compares the sexratio in first versus second and subsequent births.

For both White and Black populations, there was astatistically significant decrease in sex ratio for second

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562 J. F. X. EGAN et al.

Table1

Sexratiosatbirthin4-yeargroupsbymaternalrace/ethnicityintheUnitedStatesfrom1975to2002

Year

White

Black

Chinese

Filipino

IndianAsian

Korean

Total

Ratiom/f

Total

Ratiom/f

Total

Ratiom/f

Total

Ratio

m/f

Total

Ratiom/f

Total

Ratiom/f

1975

1978

10491596

1.058

2121821

1.028

39605

1.055

50572

1.06

4

1979

1982

11557875

1.056

2347909

1.030

52138

1.074

61792

1.07

5

1983

1986

11789564

1.055

2408164

1.031

66704

1.060

81512

1.08

1

1987

1990

12521278

1.054

2671001

1.030

85916

1.072

98603

1.08

1

1991

1994

12713788

1.052

2651501

1.031

99667

1.080

115324

1.06

4

32673

1.069

24329

1.093

1995

1998

12383309

1.050

2407735

1.031

112372

1.081

124328

1.06

9

65097

1.072

33269

1.080

1999

2002

12678892

1.050

2428415

1.032

128198

1.078

128268

1.07

4

102328

1.066

40132

1.071

Tota

l

84136302

1.053

17036546

1.030

584600

1.074

660399

1.07

3

200098

1.068

97730

1.080

Racial/ethnicdatawasnotavailableforIndianA

siansandKoreansfrom1975to1990.

m/f,

male/female.

p