title: The human sex ratio from conception to birth.
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.
authors.affiliation: Fresh Pond Research Institute, Cambridge, MA 02140; orzack@freshpond.org.
authors.forename: Steven Hecht
authors.initials: SH
authors.lastname: Orzack
authors.affiliation: Fresh Pond Research Institute, Cambridge, MA 02140;
authors.forename: J William
authors.initials: JW
authors.lastname: Stubblefield
authors.affiliation: Genzyme Genetics, Westborough, MA 01581;
authors.forename: Viatcheslav R
authors.initials: VR
authors.lastname: Akmaev
authors.affiliation: Reprogenetics, Livingston, NJ 07039;
authors.forename: Pere
authors.initials: P
authors.lastname: Colls
authors.affiliation: Reprogenetics, Livingston, NJ 07039;
authors.forename: Santiago
authors.initials: S
authors.lastname: Munné
authors.affiliation: Genzyme Genetics, Westborough, MA 01581;
authors.forename: Thomas
authors.initials: T
authors.lastname: Scholl
authors.affiliation: Oxford University, Oxford OX1 3TG, United Kingdom; and.
authors.forename: David
authors.initials: D
authors.lastname: Steinsaltz
authors.affiliation: Harvard Medical School and Brigham and Women's Hospital, Boston, MA 02115.
authors.forename: James E
authors.initials: JE
authors.lastname: Zuckerman
mesh_terms: D000028:Abortion, Induced; D000367:Age Factors; D015193:Chorionic Villi Sampling; D004622:Embryo, Mammalian / Q000502:physiology; D005260:Female; D005306:Fertilization; D006801:Humans; D007621:Karyotyping; D008297:Male; D036801:Parturition; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D027724:Reproductive Techniques, Assisted; D012744:Sex Ratio
keywords: demography; development; evolution; genetics; sex ratio
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