For a study, researchers wanted to examine the risks of maternal and perinatal outcomes in low-risk nulliparous patients undergoing expectant care based on the completed week of gestation from 39 weeks. A secondary analysis of a multicenter randomized trial of elective induction of labor at 39 weeks of gestation vs expectant management in low-risk nulliparous individuals was performed. Participants with nonanomalous neonates who were randomized to and received expectant care and had reached 39 0/7 weeks of gestation were included in the study. The gestation of delivery was classified by completed week: 39 0/7–39 6/7 (39 weeks), 40 0/7–40 6/7 (40 weeks), and 41 0/7–42 2/7 (41–42 weeks) (no deliveries after 42 2/7). Cesarean delivery and a perinatal composite were the coprimary outcomes (death, respiratory support, 5-minute Apgar score 3 or less, hypoxic-ischemic encephalopathy, seizure, sepsis, meconium aspiration syndrome, birth trauma, intracranial or subgaleal hemorrhage, or hypotension requiring vasopressor support). A maternal composite (blood transfusion, surgical intervention for postpartum hemorrhage, or intensive care unit hospitalization) was also included, as were hypertensive disorders of pregnancy, peripartum infection, and neonatal intermediate or intensive care unit admission. P<.0125 was judged statistically significant for the coprimary outcomes in multivariable analysis.

Of the 2,502 patients who received expectant management, 964 (38.5%) delivered at 39 weeks, 1,111 (44.4%) at 40 weeks, and 427 (17.1%) at 41–42 weeks. The total prevalence of medically advised delivery was 37.9%, rising from 23.8% at 39 weeks to 80.3% at 41–42 weeks. Cesarean section (17.3%, 22.0%, 37.5%; P<.001) and the perinatal composite (5.1%, 5.9%, 8.2%; P=.03) increased with 39, 40, and 41–42 weeks of gestation, respectively, while hypertensive disorders of pregnancy decreased (16.4%, 12.1%, 10.8%, P=.001). Cesarean delivery at 41–42 weeks of gestation (1.93, 1.61–2.32) and hypertensive disorders of pregnancy at 40 weeks (0.71, 0.58–0.88) and 41–42 weeks (0.61, 0.45–0.82) had significant adjusted relative risks, 95% CI (39 weeks as reference). None of the other outcomes was statistically significant. 

From 39 to 42 weeks of gestation, the frequency of medically advised induction of labor, as well as the risks of cesarean delivery, but not the perinatal composite outcome, increased considerably in expectantly treated low-risk nulliparous individuals.