Female embryos less likely to survive to birth

New research has challenged the prevailing belief that the higher proportion of male babies born in the general population results from a higher proportion of males being conceived. The conclusions suggest that embryonic death is bound up with the embryo's sex in ways that are not yet fully understood. Early embryos that are nonviable (because they show chromosomal abnormalities) and miscarried very early in pregnancy are more likely to be male, while the embryos that miscarry later in the first-trimester are more likely to be female, for reasons that remain unclear. The study of five different data sets, the largest of its kind, found that an equal number of male and female embryos are conceived (the 'primary sex ratio'), but higher female mortality (excluding abortion) in the first half of pregnancy leads to a higher percentage of males born (the 'secondary sex ratio'). The paper is published in Proceedings of the National Academy of Sciences . This study brings together five different data sets mainly from the USA, including a data on early-stage embryos, amniocentesis results from around 800,000 patients, and foetal death and live birth data from 1995 through 2004 in the USA. The study found that chromosomal abnormalities that would normally make the embryo nonviable are more common in males.