Human Females Menstruate to Eliminate Unviable Fetuses

From a female perspective, pregnancy is always a huge investment. Even more so if her species has a hemochorial placenta. Once that placenta is in place, she not only loses full control of her own hormones, she also risks hemorrhage when it comes out. So it makes sense that females want to screen embryos very, very carefully. Going through pregnancy with a weak, inviable or even sub-par fetus isn't worth it. That's where the endometrium comes in. You've probably read about how the endometriu...

 The Importance of the Tribe in Parenting

Birth—before the advent of modern medicine—often resulted in the mother’s death. Though no one knows the true figure, estimates run as high as 1 in 8. Tribes with females who could quickly relate to and trust nearby females were more likely to survive. Older females, with the wisdom of their prior birthing experiences, could care for new mothers. Women with kids could provide precious milk to a new baby if the birth mother died. Sharing and its accompanying social interactions thus prov...

 Environmental Influence on Gender

There are many well-established natural factors that bias the sex ratio of human offspring, proving that it is at least possible. The most famous is the returning-soldier effect. During and immediately after major wars, more sons are born than usual in the belligerent countries as if to replace the men that died. (This would make little sense; the men born after wars will mate with their contemporaries, not with those widowed by the war). Older fathers are more likely to have girls, but older...