RIVIERA MAYA, MEXICO — Routine induction of labor at 41 weeks is safe for women with low-risk singleton pregnancies and may decrease the risk of postterm pregnancy complications for both mother and baby, Dr. Errol Norwitz said at a meeting on obstetrics, gynecology, perinatal medicine, neonatology, and the law.
“This would affect 30% of low-risk deliveries and 10%–15% of all deliveries, so this is a big management shift,” said Dr. Norwitz, director of maternal-fetal medicine at Yale-New Haven Hospital, Conn. “It would definitely affect your practice.”
But such a change would be both cost effective and safer for mother and baby, he asserted. Recent data suggest that routine induction of labor in these women is safer than previously thought, with little or no impact on cesarean delivery rates, and that the risks of postterm birth are greater than previously thought.
Stillbirth is the greatest risk for the postterm fetus, with a fourfold increase at 43 weeks and a sevenfold increase by 44 weeks, compared with 40 weeks. Newer studies have identified other problems as well, including fetal macrosomia, meconium staining, “fetal distress,” and uteroplacental insufficiency. Neonatal encephalopathy is also a risk, with a 13-fold increase at 42 weeks, compared with 38 weeks.
But the mother is also at risk, Dr. Norwitz said. “This is an underappreciated problem. Shoulder dystocia is much more common, as is severe perineal injury, with third- and fourth-degree tears. There is also an increased risk of postpartum hemorrhage.”
In 1997, the American College of Obstetricians and Gynecologists recommended induction of labor after 43 weeks for low-risk pregnancies, but the current guidelines, issued in 2004, don't offer specific recommendations. This omission is possibly the result of concerns that labor induction is associated with an increase in the incidence of cesarean deliveries—an association that may never be conclusively proved or disproved, Dr. Norwitz said. “It would take a randomized controlled trial of 150,000 post-term pregnancies to really answer this question, and I don't think we're going to get that. We have to appreciate that the literature in this area is limited.”
The best source of data is a 2000 Cochrane Database review, which included 26 trials of various size and quality (Cochrane Database Syst. Rev. 2000;2:CD000170). Those of highest quality, Dr. Norwitz said, were two randomized controlled trials of 108 (1992) and 440 (1994) pregnancies, and a Canadian trial of almost 3,500 conducted in 1992. Both 1992 trials showed a significant decrease in cesarean rates among pregnancies induced at 41 weeks, while the 1984 study showed no significant difference in cesarean rates between the two groups. The review also concluded that routine induction of labor after 41 weeks appeared to reduce perinatal mortality.
“It appears that in multiparous women and nulliparas with a favorable cervical exam, routine induction at 41 weeks doesn't carry an increased risk of a C-section,” Dr. Norwitz said. “But in nullips with an unfavorable cervical exam, we might see the rate increase slightly. For these women, we must weigh the risk of preventing postterm complications to mom and baby with the risks of a cesarean delivery.”
Dr. Norwitz offered an algorithm for managing post-term, low-risk, singleton pregnancies:
About 50% of all pregnancies reach the 40th week. At this time, discuss the option of induction and check the cervix, but do not institute fetal surveillance. About half of the group will deliver spontaneously within the next week. For the remaining patients, offer either induction of labor or expectant management at 41 weeks.
For the women who elect continued expectant management, discuss the risks of continuing the pregnancy beyond 41 weeks and document the discussion. Institute some method of fetal surveillance to assess the baby's condition. “No single test has ever been shown to be better than another, with the exception of Doppler velocimetry alone—that has not been shown to be adequately sensitive” Dr. Norwitz said. “Most of us do twice weekly fetal testing, and one of those assessments should include an estimation of amniotic fluid volume.”
Most of these women will give birth by 42 weeks, leaving only 3%–4% of pregnancies to continue into the 43rd week. At this time, induction of labor should routinely be recommended because the increased risk of stillbirth is significant, he said.
Most women who choose induction at 41 weeks will deliver successfully, but some inductions will fail. Those women can be admitted for rupture of membranes and oxytocin, or sent home and brought back for a repeat induction in 2–3 days if the fetal testing is reassuring, Dr. Norwitz said.