It’s one of the great frustrations of obstetric medicine: humans have been reproducing for hundreds of thousands of years, and yet doctors still haven't unraveled the mystery of why some women give birth well before their babies have fully developed in the womb. Despite researchers' and physicians' best efforts, the rate of preterm births--defined as babies born before 37 weeks of gestation--climbed 30 percent from 1981 through 2006.
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