By the 30th week or so of pregnancy, most mothers-to-be probably wonder whether their babies will ever arrive. They usually will be kept waiting seven or eight more weeks, since term is defined as being from the 37th to the 42nd month of pregnancy. But between 6 and 8 percent of babies in the United States are premies, which means that they are born somewhere between the 20th and 37th week of pregnancy. According to the American College of Obstetricians and Gynecologists, preterm delivery is "the single most important problem of pregnancy."

Thanks to advances in medical technology, even babies born as early as 25 weeks can sometimes be nurtured in a hospital incubator while the infant's body systems develop outside the womb, but breathing problems, brain damage, infection, and intestinal difficulties can all befall a preterm newborn.

The cause of preterm delivery is poorly understood, but the threat appears higher in women with poor prenatal care. Women are also at heightened risk if they have: a history of preterm birth in prior pregnancies; a history of having several induced abortions; multiple pregnancy; uterine abnormalities such as a poorly formed cervix or fibroids; abdominal surgery; an infection; bleeding in the second trimester; an underweight condition; placenta previa (the placenta lies low in the uterus, partly or completely blocking the baby's exit through the cervix); early rupturing of membranes; high blood pressure or a chronic illness.

Rupturing of the membranes is an obvious sign that preterm labor may soon begin, but the signals are often much more difficult to discern. A change in vaginal discharge, pelvic or lower abdominal pressure, a low-grade backache, and abdominal cramps and contractions can often disguise preterm labor.

To stave off preterm birth, doctors may recommend bedrest, the consumption of extra fluids by mouth or intravenously, and in some cases drugs to inhibit uterine contractions. Before doing so, however, the doctor must conclude that delaying labor will pose no harm to either mother or fetus, which, in some cases, it might.

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