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Risk of maternal and fetal labor and delivery complications increase as pregnancy progresses beyond 39 weeks of gestation

The risks to infants born at 42 weeks' gestation, or postterm pregnancy, are well-documented. However, a new study reveals that infant and maternal risks begin to rise even before then. Researchers found increased rates of prolonged labor, cesarean delivery, postpartum hemorrhage, nonreassuring fetal heart rate, and other problems as early as 40 weeks of gestation among a group of fully insured pregnant women.

Researchers retrospectively studied 119,254 low-risk women who delivered babies beyond 37 weeks' gestational age from 1995 to 1999 within a managed care organization. Among these women, the rates of operative vaginal delivery (use of forceps or vacuum extraction) were 15 percent greater, 3rd or 4th degree perineal laceration 15 percent greater, and chorioamnionitis (infection of the amniotic fluid and/or placental membranes) 32 percent greater at 40 weeks compared with 39 weeks of gestation.

Also, cesarean indications for nonreassuring fetal heart rate (heart rate has repetitive decelerations from its baseline, prolonged decelerations, or decreased beat-to-beat variability) jumped 81 percent and cephalopelvic disproportion (infant's head is too large for the woman's pelvis) increased 64 percent at 40 weeks compared to 39 weeks' gestation.

Finally, rates of postpartum hemorrhage increased 21 percent, postpartum endomyometritis (infection of the uterine lining) increased 46 percent, and primary cesarean delivery increased 28 percent at 41 weeks' gestation as compared to 39 weeks. These findings underscore the need for clinical researchers to address the issue of pregnancies that progress beyond the estimated due date.

The researchers also suggest that perhaps the definition of postterm pregnancy should be earlier than 42 weeks. Their study was supported in part by the Agency for Healthcare Research and Quality (HS10856).

See "Maternal and obstetric complications of pregnancy are associated with increasing gestational age at term," by Aaron B. Caughey, M.D., M.P.P., M.P.H., Ph.D., Naomi E. Stotland, M.D., A. Eugene Washington, M.D., M.Sc., and Gabriel J. Escobar, M.D., in the February 2007 American Journal of Obstetrics & Gynecology 196, pp. 155.e1-155.e6.

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