Elective Induction at Term Reduces Perinatal Mortality Without Increasing Operative Delivery? Looking Behind the Curtain
A recent study of elective induction at term purports to show that it would reduce perinatal mortality without affecting spontaneous birth rates, although it would increase admission to a special neonatal care unit if done before 41 weeks. The study, conducted in Scotland, analyzed outcomes of 1,271,549 women carrying a singleton, head-down baby of 37 to 40 weeks gestation who gave birth between 1981 and 2007. (Forty-one weeks was considered postterm.) Women with prior cesarean, breech baby, or placenta previa were excluded. Elective induction was defined as induction with no medical indications (hypertensive or kidney disorders, thromboembolic disease, diabetes, liver disorders, pre-existing medical disorder, antenatal investigation of abnormality, suspected fetal abnormality, fetal compromise, or previous stillbirth or neonatal death), and 176,136 women met these criteria. Perinatal mortality was defined as stillbirth or death within the first month, excluding deaths associated with congenital anomalies. Outcomes were adjusted for maternal age, parity (no prior births vs. one or more prior births), time period, and birth weight.
Investigators reported outcomes by week in two ways: women electively induced compared with women not electively induced who delivered after that week and women electively induced compared with women not electively induced who delivered in or after that week. I will report outcomes according to the second method because it is less biased.
Perinatal mortality rates declined from 2.4 per 1000 at 37 weeks to 1.6 per 1000 at 41 weeks in the “not electively induced” population and varied from 0.9 to 0.6 per 1000 in the electively induced population, showing no trend, which meant that the excess
perinatal mortality rate fell from 2.3 per 1000 more deaths at 37 weeks in the “not electively induced” population to 0.9 more at 41 weeks. That would seem to clinch the argument for elective term induction were it not for one fatal flaw: investigators did not compare similar populations. They isolated a low-risk—I may even say ultra-low-risk—group of women and compared them with everyone else, including women with the high-risk conditions listed above! Finding lower perinatal mortality rates should not be surprising. It would have been extraordinary if they had not.
Even with that advantage, more babies were admitted to special or intensive care nurseries after elective induction at every week through 40 weeks, which contradicts the current belief that elective delivery at 39 weeks poses no excess risk. Excesses declined from 94 more babies per 1000 with elective induction at 37 weeks to 10 more babies per 1000 at 40 weeks. (At 41 weeks, 3 more babies per 1000 were admitted to special or intensive care in the “not electively induced” population.)
What about finding similar spontaneous vaginal birth rates? Spontaneous birth rates were, indeed, similar between groups, but more women delivered via cesarean surgery in the electively induced group. Depending on the week, 0.3 to 1.5 more women per 100 electively induced had cesareans. Spontaneous birth rates were similar because the cesarean excess was offset by an excess of instrumental vaginal deliveries at each week in the “no elective induction” group. An excess of instrumental deliveries is concerning primarily because of the increased likelihood of anal sphincter injury; however, an excess in cesarean deliveries is far more serious, carrying as it does increased likelihood of severe maternal and perinatal morbidity and mortality in both current and future pregnancies.
Furthermore, the investigators chose not to report cesarean rates according to parity. Women with a prior vaginal birth or births will be little affected by induction, but first-time mothers are a different story. Studies (see references below) comparing term elective induction with spontaneous onset report that elective induction roughly doubles the chance of cesarean with excesses ranging from 3 to 31 more women per 100 having labor end in cesarean. Three studies (Hannah et al. 1996, Kassab et al, 2011; Pavicic et al. 2009.) specifically evaluating elective induction at 41 weeks compared with expectant management for at least one more week in low-risk first-time mothers report a remarkably similar excess: 8 to 9 more cesareans per 100 women induced electively. In first-time mothers, then, the excess cesarean surgery rate was almost certainly much greater than the excess rate in the Scottish population overall.
So there you have it. Does elective induction at term save babies? We don’t know because the investigators compared apples to oranges. It certainly increases likelihood of admittance to special or intensive neonatal care through 40 weeks, an excess all the more ominous because comparison women were not all low risk. It’s also a safe bet that it substantially increases cesarean surgery rates in first-time mothers going by what other studies have found. And, again, the excess would likely have been greater even in the population overall had investigators compared low-risk women to low-risk women. Lesson learned: if you don’t look at what’s behind the curtain, you may get very misleading ideas of what is really going on.
Boulvain, M., Marcoux, S., Bureau, M., Fortier, M., & Fraser, W. (2001). Risks of induction of labour in uncomplicated term pregnancies Paediatr Perinat Epidemiol, 15(2), 131-138.
Cammu, H., Martens, G., Ruyssinck, G., & Amy, J. J. (2002). Outcome after elective labor induction in nulliparous women: A matched cohort study. Am J Obstet Gynecol, 186(2), 240-244.
Dublin, S., Lydon-Rochelle, M., Kaplan, R. C., Watts, D. H., & Critchlow, C. W. (2000). Maternal and neonatal outcomes after induction of labor without an identified indication. Am J Obstet Gynecol, 183(4), 986-994.
Ehrenthal, D. B., Jiang, X., & Strobino, D. M. (2010). Labor induction and the risk of a cesarean delivery among nulliparous women at term. Obstet Gynecol, 116(1), 35-42.
Glantz, J. C. (2005). Elective induction vs. Spontaneous labor associations and outcomes. J Reprod Med, 50(4), 235-240.
Le Ray, C., Carayol, M., Breart, G., & Goffinet, F. (2007). Elective induction of labor: Failure to follow guidelines and risk of cesarean delivery. Acta Obstet Gynecol Scand, 86(6), 657-665.
Luthy, D. A., Malmgren, J. A., & Zingheim, R. W. (2004). Cesarean delivery after elective induction in nulliparous women: The physician effect. Am J Obstet Gynecol, 191(5), 1511-1515.
Macer, J. A., Macer, C. L., & Chan, L. S. (1992). Elective induction versus spontaneous labor: A retrospective study of complications and outcome. Am J Obstet Gynecol, 166(6 Pt 1), 1690-1696; discussion 1696-1697.
Maslow, A. S., & Sweeny, A. L. (2000). Elective induction of labor as a risk factor for cesarean delivery among low-risk women at term. Obstet Gynecol, 95(6 Pt 1), 917-922.
Prysak, M., & Castronova, F. C. (1998). Elective induction versus spontaneous labor: A case-control analysis of safety and efficacy. Obstet Gynecol, 92(1), 47-52.
Seyb, S. T., Berka, R. J., Socol, M. L., & Dooley, S. L. (1999). Risk of cesarean delivery with elective induction of labor at term in nulliparous women. Obstet Gynecol, 94(4), 600-607.
Vahratian, A., Zhang, J., Troendle, J. F., Sciscione, A. C., & Hoffman, M. K. (2005). Labor progression and risk of cesarean delivery in electively induced nulliparas. Obstet Gynecol, 105(4), 698-704.
van Gemund, N., Hardeman, A., Scherjon, S. A., & Kanhai, H. H. (2003). Intervention rates after elective induction of labor compared to labor with a spontaneous onset. A matched cohort study. Gynecol Obstet Invest, 56(3), 133-138.
Vardo, J. H., Thornburg, L. L., & Glantz, J. C. (2011). Maternal and neonatal morbidity among nulliparous women undergoing elective induction of labor. J Reprod Med, 56(1-2), 25-30.
Vrouenraets, F. P., Roumen, F. J., Dehing, C. J., van den Akker, E. S., Aarts, M. J., & Scheve, E. J. (2005). Bishop score and risk of cesarean delivery after induction of labor in nulliparous women. Obstet Gynecol, 105(4), 690-697.
Yeast, J. D., Jones, A., & Poskin, M. (1999). Induction of labor and the relationship to cesarean delivery: A review of 7001 consecutive inductions Am J Obstet Gynecol, 180(3 Pt 1), 628-633.