The human sex ratio is the ratio of males to females in a population. Global sex ratio records at birth consistently suggest that boys naturally outnumber girls at birth. Researchers at Harvard University and Oxford University analyzed the largest dataset from United States to explain the human sex ratio, which has been poorly understood until now. Their findings suggest that boys and girls are equal in number at conception, but more female fetuses die during pregnancy, leading to a slightly higher number of males being born.
Researchers analyzed the sex ratios of fetuses at different gestational ages, including three-to-six-day-old embryos produced by assisted reproductive technologies, induced abortions, chorionic villus sampling, amniocentesis, and United States census records of fetal deaths and live births. They found that the human sex ratio is equal at conception, but that during gestation, there were certain times when male embryos were more likely to die than females.
Scientists found that in the first week after conception there tended to be more abnormal male embryos than female embryos, while during the next 10 to 15 weeks, females faced a higher risk of mortality in the womb. Later in pregnancy, male miscarriages were higher in number than female miscarriages, particularly at weeks 28 to 35. But overall, the study showed that more females died in the womb than males.