Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2015 Apr 21;112(16):E2102-11.
doi: 10.1073/pnas.1416546112. Epub 2015 Mar 30.

The human sex ratio from conception to birth

Steven Hecht Orzack  1 , J William Stubblefield  2 , Viatcheslav R Akmaev  3 , Pere Colls  4 , Santiago Munné  4 , Thomas Scholl  3 , David Steinsaltz  5 , James E Zuckerman  6
Affiliations

The human sex ratio from conception to birth

Steven Hecht Orzack et al. Proc Natl Acad Sci U S A. .

Abstract

We describe the trajectory of the human sex ratio from conception to birth by analyzing data from (i) 3- to 6-d-old embryos, (ii) induced abortions, (iii) chorionic villus sampling, (iv) amniocentesis, and (v) fetal deaths and live births. Our dataset is the most comprehensive and largest ever assembled to estimate the sex ratio at conception and the sex ratio trajectory and is the first, to our knowledge, to include all of these types of data. Our estimate of the sex ratio at conception is 0.5 (proportion male), which contradicts the common claim that the sex ratio at conception is male-biased. The sex ratio among abnormal embryos is male-biased, and the sex ratio among normal embryos is female-biased. These biases are associated with the abnormal/normal state of the sex chromosomes and of chromosomes 15 and 17. The sex ratio may decrease in the first week or so after conception (due to excess male mortality); it then increases for at least 10-15 wk (due to excess female mortality), levels off after ∼20 wk, and declines slowly from 28 to 35 wk (due to excess male mortality). Total female mortality during pregnancy exceeds total male mortality. The unbiased sex ratio at conception, the increase in the sex ratio during the first trimester, and total mortality during pregnancy being greater for females are fundamental insights into early human development.

Keywords: demography; development; evolution; genetics; sex ratio.

PubMed Disclaimer

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
The relationship between conception age and cohort sex ratio estimated from induced-abortion data. Observed sex ratios and estimated regression for chromatin (●) and for karyotype (○) data (Table 6). A dashed line denotes a sex ratio of 0.5.
Fig. 2.
Fig. 2.
The relationship between conception age and cohort sex ratio estimated from CVS data. Observed cohort sex ratio (with 95% confidence limits) and the estimated regression (Table 8). Fractional ages are rounded to the nearest integer. A dashed line denotes a sex ratio of 0.5.
Fig. 3.
Fig. 3.
The relationship between conception age and cohort sex ratio estimated from amniocentesis data. Observed cohort sex ratio (with 95% confidence limits) and the estimated regression (Table 10). Fractional conception ages are rounded to the nearest integer. A dashed line denotes a sex ratio of 0.5.
Fig. 4.
Fig. 4.
The relationship between conception age and cohort sex ratio estimated from US fetal deaths and live births for 1995–2004 (combined). Observed cohort sex ratio (with 95% confidence limits). Conception age is based on the date of the last menstrual period; 18 denotes ≤18 wk. A dashed line denotes a sex ratio of 0.5.
Fig. 5.
Fig. 5.
The trajectory of the cohort sex ratio from conception to birth. ALL and NORMAL denote the total and normal sex ratio estimates based on ART embryos (Table 1), respectively, CVS denotes the estimated sex ratio trend based on CVS data (Table 8), ABORTION denotes the estimated trend based on induced abortions sexed via karyotype (Table 6), AMNIO denotes the estimated trend based on amniocentesis data (Table 10), and FDN denotes the trend of cohort sex ratio based on US fetal deaths and live births. A dashed line denotes a sex ratio of 0.5.
See this image and copyright information in PMC

Comment in

  • The human prenatal sex ratio: A major surprise.
    Austad SN. Austad SN. Proc Natl Acad Sci U S A. 2015 Apr 21;112(16):4839-40. doi: 10.1073/pnas.1505165112. Epub 2015 Apr 6. Proc Natl Acad Sci U S A. 2015. PMID: 25848060 Free PMC article. No abstract available.
  • The human sex ratio at conception.
    James WH, Grech V. James WH, et al. Early Hum Dev. 2020 Jan;140:104862. doi: 10.1016/j.earlhumdev.2019.104862. Epub 2019 Sep 3. Early Hum Dev. 2020. PMID: 31492545

References

    1. Graunt J. Natural and Political Observations Made Upon the Bills of Mortality. Martyn; London: 1662.
    1. Campbell RB. John Graunt, John Arbuthnott, and the human sex ratio. Hum Biol. 2001;73(4):605–610. - PubMed
    1. Nichols JB. The numerical proportions of the sexes at birth. Mem Am Anthropol Assoc. 1907;1(4):247–300.
    1. Klug WS, Cummings MR, Spencer C. Concepts of Genetics. 8th Ed Pearson/Prentice Hall; Upper Saddle River, NJ: 2006.
    1. Cunningham FG, et al. 2010 Williams Obstetrics. 23rd Ed McGraw-Hill Medical; New York: 2010.

Publication types

LinkOut - more resources