This passage from pg. 135 to 137 of Zimmer’s Parasite Rex simply “blew my mind”:

“…When a fertilized egg tumbles down into the uterus and tries to implant itself, it encounters an army of macrophages and other immune cells. The new embryo doesn’t have the same proteins on its cells as its mother, which ought to trigger the immune cells to destroy it. The fetus faces the same troubles as a rapeworm or a blood fluke, and it evades its mother’s immune system in much the same way….

If you think of parasitism in terms of Dawkins’s definition of genetic interests, then a fetus is a sort of half-parasite. It shares half its genes with its mother, and the rest belong to its father. Both mother and father have an interest, evolutionarily speaking, in seeing the fetus get born and live a healthy life. But some biologists have argued that parents also have strong conflicts on how the fetus grows. As it develops, it builds its placenta and a network of vessels to draw nourishment out of its mother. It knocks out its mother’s control over her blood vessels near the uterus, so that she can’t restrict the flow of blood to the fetus. It even releases chemicals to raise the concentration of sugar in her blood. But if the mother lets her child take too much, it might take a serious toll on her health. She might not be able to take care of her other children, and it might even threaten her ability to have any more. In other words, the fetus threatens her genetic legacy. Research suggests that mothers struggle against their fetus, releasing counteracting chemicals of their own.

While a fetus can take a heavy toll on its mother, how fast it grows will have no effect on its father’s health. It’s in his genetic interest for the fetus to grow as fast as possible. This conflict plays out within the fetus itself. Research on animals has shown that the genes a fetus inherits from its father and mother do different things, particularly in the trophoblasts. The maternal genes try to slow down the growth of the fetus, to control this parasite within her. Meanwhile, the paternal genes clamp down on these maternal genes and silence them, letting the fetus grow faster and draw more energy from its host.”

In short, we are riven by conflicts, even before we’re born!