Moms-to-be generally spend a good portion of their pregnancy preparing for labor and the subsequent delivery, learning what to expect, how to make it "easier" and what their options are.

The average labor for a first-time mom is 12 to 18 hours, with labors for subsequent pregnancies averaging six to eight hours. What happens, though, when labor is prolonged? What are the causes of prolonged labor, and what should you know about it?

There are three stages of labor. The first stage is defined as the onset of labor to full dilation of the cervix. This is followed by the second stage, from full dilation to delivery, and the third stage begins after delivery of the infant and ends with delivery of the placenta.

"Doctors use the term 'abnormal progress of labor' when a woman is having trouble dilating during the first stage," says Dr. George K. Tweddel, Chief of Obstetrics at Somerset Medical Center in Somerville, N.J. "For a woman delivering one baby, more than 20 hours during the first stage of labor is considered abnormal progress of labor."

Prolonged labor can have many causes, but the main cause is that the fetus is too large to fit easily through the mother's pelvis. "This is called 'pelvic disproportion,'" Dr. Tweddel says. "Other impediments to a smooth first stage of labor are that the fetus is in an unusual position, such as face up, and that the uterine contractions are too weak."

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