CURIOUS: Nearly 4 percent of US babies born before full-term without medical reason.

Researchers reviewed data linking birth certificates with hospital records for all births in California, Missouri and Pennsylvania between 1995 and 2009. The three states represent approximately 20 percent of all U.S. births and encompass a wide range of sociodemographic characteristics and geographic locations.

“Our study showed that early elective deliveries made up more than 3 percent of U.S. births each year over the past 20 years. This may seem to be a small number, but with 4 million births a year in the U.S., each percentage point represents 40,000 babies,” said Kozhimannil. “In addition, we showed that there are important sociodemographic differences in the chances a pregnant woman has an early elective cesarean or an early elective induction of labor.”