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One out of every ten pregnant women gives birth before term -- the end of the 37th week.
Only 30 percent of these deliveries involve more than one baby. Another 20 percent have risk factors, such as previous preterm labor and bleeding during the second trimester. However, half of those who deliver prematurely lack any major risk factor. There are still many unanswered questions about why preterm delivery occurs.
Dr. Jay D. Iams, M.D., professor of obstetrics and gynecology at Ohio State University College of Medicine in Columbus, compares most cases to heart disease, which is caused by a combination of factors, such as a poor diet, lack of exercise and smoking. Similarly, various factors during pregnancy can cumulatively put strain on the body and lead to early labor.
Sue Wallace, R.N., "Sooner than expected?" Parents, September 1998, pp. 127-128