Pregnancy-a conflict

In the 1970's, Dr. Trivers argued that families create an evolutionary conflict. Natural selection should favor parents who can successfully raise the most offspring. For that strategy to work, they
can't put too many resources into any one child. But the child's chances for reproductive success will increase as its care and feeding increase. Theoretically, Dr. Trivers argued, natural selection could favor genes that help children get more resources from their parents than the parents want to
give.
As Dr. Haig considered the case of pregnancy, it seemed like the perfect arena for this sort of conflict. A child develops in intimate contact with its mother. Its development in the womb is crucial to its long-term health. So it was plausible that nature would favor genes that allowed fetuses to draw more resources from their mothers. A fetus does not sit passively in its
mother's womb and wait to be fed. Its placenta aggressively sprouts blood vessels that invade its mother's tissues to extract nutrients. Meanwhile, Dr. Haig argued, natural selection should favor mothers who could restrain these incursions, and manage to have several surviving offspring carrying on their genes. He envisioned pregnancy as a tug of war. Each side pulls hard, and yet a
flag tied to the middle of the rope barely moves.