Pregnant women in America are giving birth a week earlier than they were in 1992.
From 10,000 BC to 1992, it's pretty much been the same," California Watch reporter Nathanael Johnson said.
The normal length of pregnancy is 40 weeks but according to researchers and data at the Centers for Disease Control and Prevention, the average is now 39 weeks, a full seven days earlier.
"What Nate has found is undoubtedly true, there is a decrease, a general decrease of the average gestational age of birth in America," Dr. Elliot Main said.
Main is chairman of Obstetrics and Gynecology at California Pacific Medical Center in San Francisco. He is also director of the California Maternal Care Collaborative, a group that constantly evaluates best practices for delivering babies.
Main says a full term pregnancy is generally defined as 39 to 41 weeks. He believes the week of pregnancy lost in the last 20 years is probably not having a significant impact of the health of most infants.
"What we are telling all of our mothers is that you really ought to wait until 39 weeks if everything is going fine and not intervene earlier than nature would intend," Main said.
One reason for the increase in early deliveries may be because more women are actually scheduling childbirth for convenience. One study of nearly 18,000 deliveries in 2007 showed 9.6 percent of early births were scheduled C-sections.
Main says that may explain some of the early deliveries, others may be simply a matter of better care.
"We are doing a much better job of identifying the true do date with ultrasound so we no longer have mothers going 3 or 4 weeks past their due date," he said.
A number of medical groups are now encouraging doctors to not deliver babies early, including the American Congress of Obstetricians and the March of Dimes.
"The interesting thing is that this is something that kind of everybody has known about for a long time and now they are finally saying, 'OK, we should do something about this,'" Johnson said.
Written and produced by Ken Miguel