Why Nighttime Is Labor Intensive

Ever notice how pregnant women tend to go into labor at the most inconvenient time possible? Don't blame mom: Studies suggest that babies, not mothers, get the birth process going, says Cornell University's Peter W. Nathanielsz, M.D., Ph.D.

Until the very end of prey nancy, a woman's supply of the hormone progesterone inhibits her uterus from contracting. But as a fetus nears maturity, its brain revs up pro-auction of the hormone cortisol, in effect telling its mother, "Okay, I'm ready." This cortisol glut dampens mom's progesterone production and boosts her estrogen levels, readying her for birth, Nathanielsz reports in American Scientist.

Unfortunately, the daily nocturnal peak in a mother's oxytocin—the last prebirth hormonal signal--makes it likely that contractions will begin after dark. It's a cruel artifact of evolution, Nathanielsz says: "It was safer for our ancestors to have babies at night because predators weren't around."

Edited by Peter Doskoch