Two generations ago babies were born pretty much spontaneously, around the clock. But today in the U.S., about half of all births are cesarean sections prescheduled by Mom or deliveries induced by doctors concerned about the mother's or baby's health. These medical procedures have skewed the days of the week, and hours of the day, during which those little bundles of joy arrive.

The procedures dominate because more than 98 percent of infants are born in a hospital, despite what seems to be the rising popularity of home births. Far more babies now arrive on weekdays than on weekends, most between 8 A.M. and 6 P.M. “We can't schedule spontaneous labor, obviously,” says Neel Shah, a physician and professor at Harvard Medical School. “But we can schedule delivery.”

Credit: Nadieh Bremer and Zan Armstrong; Source: FiveThirtyEight, from data supplied by U.S. Social Security Administration (week data); Centers for Disease Control and Prevention (minute and hour data)