There is a weird evolutionary arms-race going on about the placenta. There are some genes that, via epigenic mechanisms, only activate when inherited from a parent of specific sex.

There are genes that activate when inherited from the father, and these genes make the placenta... bigger, stronger, more aggressive, more invasive, to make the baby big and strong, possibly at the expense of the mother and her future children (which may not be by the same father).

Then there are genes that activate when inherited from the mother, which do the opposite, and try to keep the placenta in check, to minimize the risk to the mother and keep the uterus in good condition for future children.

Obviously, you can't go too far in either direction. But over the span of evolution, the balance was shifting here and there, and the genes kept accumulating. Now we carry lot of useless baggage that cancels itself out in our genome.