Aaron B. Caughey, MD, PhD, said at the 2020 virtual meeting of the American College of Obstetricians and Gynecologists.
For much of the 20th century, term gestation has been defined as 37 weeks and beyond, said Dr. Caughey, of Oregon Health & Science University, Portland. He noted several studies showing a U-shaped distribution in neonatal outcomes during the period from 37 weeks to 41 weeks for some outcomes, including Apgar scores. However, respiratory outcomes in a study from 2008 showed an increase, with meconium stained amniotic fluid increasing from 2.27% at 37 weeks to 10.33% at 41 weeks, and meconium aspiration increasing from 0.07% at 37 weeks to 0.27% at 41 weeks.
Late-term induction may carry more risk
The study “that really got everyone’s attention” in terms of neonatal outcomes was published in 2009 in the New England Journal of Medicine. The cohort study included 24,077 elective cesarean deliveries between 37 and 42 weeks and reviewed a range of neonatal outcomes based on gestational age.
The rate of any adverse outcome decreased from 37 weeks to 39 weeks, “but then started going back up again,” Dr. Caughey said. He reviewed data from another study that factored in stillbirth and the risk of expectant management based on gestational age. A composite risk of perinatal death with expectant management was 15.4 deaths per 10,000 cases at 37 weeks and 39 weeks, but increased to 19.9 at 42 weeks.
“The morbidity appears to have a U-shaped distribution and the mortality seems to favor delivery at 39 weeks,” he said.
When it comes to induction of labor, medically indicated vs. nonmedically indicated does matter, Dr. Caughey said. Factors not considered a medical indication include impending macrosomia, increased risk for developing preeclampsia or intrauterine growth retardation, and a favorable cervix, he noted.
“For indicated induction of labor, the risks and benefits of induction of labor vs. expectant management have been considered and weighed in by the field of experts that care for pregnant women,” he said. With nonmedically indicated induction, experts “either decided that risks and benefits don’t favor induction of labor, or we haven’t come down hard on what the protocol might be.
“It is important to consider the risks and benefits,” said Dr. Caughey. The factors you want to include are neonatal outcomes, maternal preferences, and doctor preferences. However, “we want to be thoughtful about this intervention,” because of the association of higher costs and increased risk of cesarean with induction of labor.
As for timing of induction of labor, certain conditions favoring early-term induction include preeclampsia and gestational hypertension, chronic hypertension, diabetes, intrauterine growth restriction, nonreassuring fetal testing, cholestasis, placenta previa or accreta, and twins.
Data support value of 39 weeks
As for late-term induction of labor, “at 41 weeks it is pretty clear that neonatal outcomes would be improved by delivery,” he said. Historically, clinicians have raised concerns about the increased risk of cesarean delivery following induction of labor, but this risk has not been borne out in recent studies. Dr. Caughey said. However, in the findings from the ARRIVE trial, a large study of 6,106 women who were randomized to induction or labor or expectant management at 39 weeks, “they found a reduction in their risk of cesarean delivery compared to expectant management (18.6% vs. 22.2%). Rates of preeclampsia also were lower among induced women, while rate of chorioamnionitis, postpartum hemorrhage, and intensive care were similar between the groups. The researchers did not find significant differences in perinatal outcomes.
Dr. Caughey and colleagues conducted a systematic review of cesarean risk and induction of labor, and found a risk ratio of 0.83, similar to the ARRIVE trial. “The data suggest a consistently reduced risk for cesarean delivery with the induction of labor.”
However, “I would caution us to be thoughtful about research protocols vs. actual practice,” he said. “You must think about the environment.” The latent phase of labor can continue for a long time after induction, and patience is called for, he emphasized.
Dr. Caughey said that despite the ARRIVE trial and other studies, 39 weeks should not necessarily be the new standard for induction of labor. “The proportion of women impacted is dramatically different, if you would be inducing every woman at 39 weeks, that would be 60% to 70%,” which could have a great impact on resources.
Based on current research, early-term induction of labor at 37 weeks “is a bad idea without indication,” said Dr. Caughey. Induction at 41 weeks (sometimes considered post term) is the current ACOG recommendation and is associated with improved outcomes.
Induction of labor at full term (39-40 weeks) depends in part on the environment, and is not a violation of standard of care, he said. “Evidence is evolving, and individual hospitals are trying to figure this out.”
Cesarean data are convincing, at least in some settings, he said. However, “we need more global trials and different medical settings” to determine the optimal time for induction of labor.
Consider maternal preferences and characteristics
During a question-and-answer session, Dr. Caughey was asked whether all women should be offered induction of labor at 39 weeks.
“I think it is OK if your entire health system has agreed to offering, to have that shared medical decision making, but you need to have careful conversation to make sure you have the resources,” he noted. Also, he said he believed clinicians should respond to women as they request labor induction at 39 weeks.
In response to a question about induction of labor in obese women, he noted that women with a body mass index greater than 35 kg/m2 are not equally successful with induction of labor. “We know they have a higher risk of cesarean delivery,” however, “it has been demonstrated that they have the same potential benefits of reduced risk of cesarean.”
As for factoring in the Bishop score to determine a favorable or unfavorable cervix, Dr. Caughey noted that women with a favorable cervix are more likely to go into labor on their own, while those with an unfavorable cervix may benefit from cervical ripening.
Dr. Caughey had no financial conflicts relevant to this talk, but disclosed serving as a medical adviser to Celmatix and Mindchild, as well as an endowment to his academic department from Bob’s Red Mill, an Oregon-based whole grain foods manufacturer.