Forget fashionably late

CHILDREN grow up so fast. And now they’re getting an earlier start on the process. The most common length of pregnancy in the U.S. is now 39 weeks, down from 40, March of Dimes researchers have found. Forty weeks has traditionally been considered the benchmark for a full-term birth.

Analyzing data from the National Center for Health Statistics on all live U.S. births in 1992, 1997 and 2002, researchers identified a notable overall shift toward shorter gestational periods, with more births at 34 to 39 weeks and fewer at 40 and longer. The shift can be attributed in part to increasing rates of Caesarean sections and induced births and an increasing reluctance among physicians to let pregnancies go past term.

“I would say it’s probably more of an evolution of practice than an evolution of the species,” says Dr. Diane Ashton, associate medical director of the March of Dimes.

Consistent with this, the study found a 21% decrease in births at week 40 or longer.

Medically, the ideal gestational period, assuming there’s no extenuating medical condition, is 39 to 40 weeks, Ashton says, although 37 weeks and beyond is considered full term. The study was published in March as a supplemental edition of the journal Seminars in Perinatology.