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Examining the Evidence

What is the optimal timing of elective cesarean delivery at term?

April 2009 · Vol. 21, No. 04

Approximately 1.3 million cesarean deliveries are performed each year in the United States—40% of them repeat procedures. As the number of cesarean deliveries continues to rise, the timing of elective cesarean delivery—including the increasing percentage of repeat cesareans—gains even more importance. The study by Tita and colleagues focuses on this population.

Unless fetal lung maturity has been confirmed, elective cesarean delivery before 39 weeks’ gestation is associated with a higher rate of neonatal respiratory problems. In this observational study, performed at 19 US centers from 1999 to 2002, and funded by the National Institutes of Health, infants of women who underwent elective cesarean delivery at 37 or more weeks’ gestation were assessed for the primary outcome—a composite neonatal outcome that included the occurrence of any of the following:

  • death
  • respiratory distress syndrome
  • transient tachypnea of the newborn
  • hypoglycemia
  • newborn sepsis
  • confirmed seizures
  • necrotizing enterocolitis
  • hypoxic–ischemic encephalopathy
  • cardiopulmonary resuscitation or ventilator support within 24 hours of birth
  • umbilical cord–blood arterial pH <7.0
  • 5-minute Apgar score of 3 or below
  • admission to NICU
  • hospitalization for 5 or more days.

Of 13,258 elective cesarean deliveries performed at term, 35.8% occurred before 39 completed weeks of gestation (6.3% at 37 weeks, 29.5% at 38 weeks) and 49.1% at 39 weeks. Women who delivered before 39 weeks were more likely to be married, white, and to have initiated prenatal care early.

Compared with infants delivered at 39 weeks, those born at 37 to 38 weeks’ gestation had a greater risk of the primary (composite) outcome. At 37 weeks, the adjusted odds ratio (OR) was 2.1 (95% confidence interval [CI], 1.7–2.5). At 38 weeks, the adjusted OR was 1.5 (1.3–1.7).

The authors estimated that, at 37 weeks’ gestation, postponing elective delivery until 39 weeks might prevent 48% of cases of the primary outcome; this percentage was estimated to be 27% at 38 weeks’ gestation.

Patient preference determines timing in some cases

Cesarean delivery accounts for almost one third of US births, and most women who have had such a delivery opt to repeat it in their next pregnancy. As an editorial accompanying this article points out, women in this study who delivered before 39 weeks were more likely to be private patients and had likely asked their own obstetrician to perform the delivery.1 Obstetricians who wish to promote patient satisfaction are likely to honor such a request, recognizing that waiting until later would increase the likelihood of labor, which would exclude the possibility of an elective procedure.

Limitations of the study

Because this study lacked data about testing for fetal lung maturity, it is unclear whether the higher rate of adverse outcomes with elective cesarean delivery before 39 weeks could be explained by failure to assess for fetal lung maturity.

It also appears that the delay of delivery to 39 weeks or beyond may be associated with an increased risk of stillbirth. In other populations, this risk has been estimated to be roughly 0.5 of every 1,000 births for each advancing week of gestation.


Obstetricians and their patients should weigh the known risks of elective cesarean delivery before 39 weeks’ gestation against the small risk of late stillbirth. At the same time, it is important to factor in the patient’s preferences about when delivery occurs and who performs it.—ANDREW M. KAUNITZ, MD


1. Greene MF. Making small risks even smaller [editorial]. N Engl J Med. 2009;360:183-184.

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