Female Embryos Are More Likely To Die Off During First Trimester

Previous research has suggested that male embryos are more likely to die off during pregnancy. Now, new findings published in the journal of the Proceedings of the National Academy of Sciences (PNAS) have found new evidence that change the perspective on that.

Researchers found that the number of embryos very early on in the pregnancy that die off are more likely to be male than female. However, as pregnancy progresses on into the first trimester, those that die off are more likely to be female.

"It's important to study male-female differences in the womb because they underlie, in part, the profound differences we see between males and females at birth and thereafter," biologist Steven Orzack at the Fresh Pond Research Institute in Cambridge, Massachusetts, said via HealthDay.

For the study, researchers examined five different data sets that included information on early-stage embryos, amniocentesis results from close to 800,000 patients, as well as foetal death and live birth from 1995 through 2004 in the United States.

Findings revealed that during the first half of the pregnancy, there is a higher female mortality rate, resulting in a higher number of male births.

While the study found that chromosomal abnormalities were higher in male embryos, this did not deter them from outliving female counterparts, suggesting that something else is going on to increase mortality risk in the females.

Of course, researchers are still learning just what that something is. The highest mortality risk for female embryos seems to be during the first trimester. By 20 weeks and thereafter, things seem to level off in female embryos.

"The higher female embryo mortality during pregnancy suggests further areas of potential investigation: we know that sex ratio can be influenced by environmental pollution and by maternal stress, but little is understood about the mechanism," study co-author and Professor David Steinsaltz of Oxford University's Department of Statistics said, via The University of Oxford. "Knowing that there are different sex biases in different periods of pregnancy could be an important part in helping to sharpen our picture of what happens when, and more generally how the fate of an embryo is determined.

"Changes in sex ratio can have enormous social ramifications: there are parts of the world where sex ratios are being distorted by parents intentionally choosing boys, and changes are introduced perhaps inadvertently by reproductive technologies. We need to understand better how the human sex ratio is biologically determined, in order to understand how our manipulations might affect it."