Happy Mother's Day, Mom! Sorry I Tried to Kill You.
Here's something I found on William Saletan's Human Nature column at Slate a couple of months ago. I figured I'd save it for today, though:
Evidence is growing that pregnancy is a fight for survival between woman and fetus. Examples: 1) The fetus makes its placenta send blood vessels into the woman to sap her nutrients. 2) To force more blood into the placenta, the fetus injects the woman with a protein that impairs her ability to fix her damaged blood vessels, thereby afflicting some women with pre-eclampsia (very high blood pressure). 3) In mice, the male transmits a gene to the fetus to make it grow larger in the womb, but the female transmits a gene to block this effect. 4) Pregnancy kills more than half a million women per year, which seems not to make evolutionary sense. Best available explanation: Natural selection designed fetuses to hog their mothers' resources, but designed women to thwart this so they'll have enough resources to produce other surviving offspring.
(emphasis in original, entire column here)