When genes fight

For those of you just tuning in: on a genetic level, all parental investments in their offspring are equally valuable.  From the offspring’s perspective investing in them is two to four times as good as investing in their sibling (depending on if they’re half or full siblings).  The fight to get the amount of resources they think they deserve is parent-offspring conflict

One manifestation of parent offspring conflict is weaning conflict, where offspring would like to keep getting nutrients with no effort and their mom would really like them to go out and get a job.

But it can start before then.  Conflict over exactly how much nutrition a fetus should get may contribute to preeclampsia (high blood pressure during pregnancy) and insulin resistance (PDF).  This is not just about mothers not wanting to give up nutrients- more nutrition leads to larger babies, which is almost always good for the baby but kind of hard on the person forcing it out through their vagina.  There is even speculation that the human custom of making a thick uterine lining (nutritionally expensive) only to flush it away each month evolved as self defense against a placenta that would otherwise invade your uterus like ivy invades bricks.

This is your uterus on babies.
This is your uterus on babies.

Creepy, yes.  Abhorrent, at the level of individuals.  But totally logical and predictable from the level of a gene.