Advertisement

Induction of labor in a contemporary obstetric cohort

Published:March 26, 2012DOI:https://doi.org/10.1016/j.ajog.2012.03.014

      Objective

      We sought to describe details of labor induction, including precursors and methods, and associated vaginal delivery rates.

      Study Design

      This was a retrospective cohort study of 208,695 electronic medical records from 19 hospitals across the United States, 2002 through 2008.

      Results

      Induction occurred in 42.9% of nulliparas and 31.8% of multiparas and elective or no recorded indication for induction at term occurred in 35.5% and 44.1%, respectively. Elective induction at term in multiparas was highly successful (vaginal delivery 97%) compared to nulliparas (76.2%). For all precursors, cesarean delivery was more common in nulliparas in the latent compared to active phase of labor. Regardless of method, vaginal delivery rates were higher with a ripe vs unripe cervix, particularly for multiparas (86.6-100%).

      Conclusion

      Induction of labor was a common obstetric intervention. Selecting appropriate candidates and waiting longer for labor to progress into the active phase would make an impact on decreasing the national cesarean delivery rate.

      Key words

      The induction rate in the United States has more than doubled from 9.5% of all deliveries in 1990 to 22.5% in 2006.
      • Martin J.A.
      • Hamilton B.E.
      • Sutton P.D.
      • Ventura S.J.
      • Menacker F.
      • Kirmeyer S.
      Births: final data for 2006.
      • Zhang J.
      • Yancey M.K.
      • Henderson C.E.
      US national trends in labor induction, 1989-1998.
      The increasing induction rate may be due in part to increased rates of complications such as gestational diabetes and preeclampsia as a result of increasing maternal age and body mass index (BMI) in the current obstetric population.
      • Cleary-Goldman J.
      • Malone F.D.
      • Vidaver J.
      • et al.
      Impact of maternal age on obstetric outcome.
      • Villamor E.
      • Cnattingius S.
      Interpregnancy weight change and risk of adverse pregnancy outcomes: a population-based study.
      Another key contributor, however, has been a rise in elective induction, when there is no medical or obstetrical indication for delivery.
      • Lydon-Rochelle M.T.
      • Cardenas V.
      • Nelson J.C.
      • Holt V.L.
      • Gardella C.
      • Easterling T.R.
      Induction of labor in the absence of standard medical indications: incidence and correlates.
      Induction of labor has been implicated in an increased risk of cesarean delivery in some studies, while other observational studies and small randomized controlled trials have demonstrated a decreased risk of cesarean and potentially improved neonatal outcomes with elective delivery.
      • Ehrenthal D.B.
      • Jiang X.
      • Strobino D.M.
      Labor induction and the risk of a cesarean delivery among nulliparous women at term.
      • Caughey A.B.
      • Sundaram V.
      • Kaimal A.J.
      • et al.
      Systematic review: elective induction of labor versus expectant management of pregnancy.
      Elective inductions may be preferentially performed in healthy pregnancies and for women who have a more favorable Bishop score, factors that typically translate to better maternal and neonatal outcomes. Yet the evidence of the benefits of elective induction <41 0/7 weeks of gestation is insufficient, since many of these studies have been limited by methodological deficiencies.
      • Caughey A.B.
      • Sundaram V.
      • Kaimal A.J.
      • et al.
      Systematic review: elective induction of labor versus expectant management of pregnancy.
      These discrepant results also suggest that the success of induction and maternal and perinatal outcomes may differ depending on the indication for induction, as well as on cervical readiness. The purpose of this study is to describe in detail the maternal and obstetrical characteristics of induction of labor in a recent obstetrical cohort in the United States, including precursors and methods for induction of labor and associated vaginal delivery rates.

      Materials and Methods

      The Consortium on Safe Labor was a study conducted by the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, and has been described in detail elsewhere.
      • Zhang J.
      • Troendle J.
      • Reddy U.M.
      • et al.
      Contemporary cesarean delivery practice in the United States.
      In brief, this was a retrospective cohort study involving 228,668 deliveries from 2002 through 2008 from 12 clinical centers and 19 hospitals representing 9 American College of Obstetricians and Gynecologists (ACOG) US districts. Institutional review board approval was obtained by all participating institutions. The population was then standardized by assigning a weight to each subject using ACOG district, maternal race/ethnicity, parity, and plurality based on 2004 National Natality data.
      • Zhang J.
      • Troendle J.
      • Reddy U.M.
      • et al.
      Contemporary cesarean delivery practice in the United States.
      • Martin J.A.
      • Hamilton B.E.
      • Sutton P.D.
      • Ventura S.J.
      • Menacker F.
      • Kirmeyer S.
      Births: final data for 2004.
      To limit overcontribution by women who had multiple pregnancies during the study period, we included the first pregnancy if a woman had >1 pregnancy in the present study.
      Induction of labor was a predefined variable when either the patient's electronic medical record indicated that there was an induction and/or a method or start time for induction was recorded in the patient's chart. There was a separate variable for labor augmentation. We defined attempts at vaginal delivery to include all women with at least 2 vaginal examinations in the labor progression database. We established 3 categories of precursors for induction: “indicated,” “elective,” and “no recorded indication.” The indication for induction was used to identify the precursors for delivery and classified using the following hierarchy. First, women with premature rupture of membranes (ROM) were always classified as such. Thus, if a woman undergoing induction of labor had both premature ROM and another indication, she was only counted once in the premature ROM category. Second, we supplemented the indication for induction with all potential maternal, fetal, or obstetric complications of pregnancy, which were then included in the “indicated” category. For example if a woman had an induction and no indication was recorded but the pregnancy was complicated by preeclampsia, then she was included in the “indicated” precursor “preeclampsia.” A woman could have >1 pregnancy condition in the “indicated” category. We also included in the “indicated” category those women who were admitted to labor and delivery for an unspecified fetal or maternal reason. Third, if a site indicated that the induction was elective; no other indications for induction were provided; and there were no other obstetric, fetal, or maternal conditions complicating the pregnancy, then the precursor for induction was designated “elective.” If a delivery indication was noted as postdates or postterm with no other indications listed but <41 weeks of gestation, these were also coded as elective. Finally, the “no recorded indication” category encompassed all inductions as identified by the site with no other obstetric, fetal, or maternal conditions of the pregnancy, including if no reason for induction was provided. Method of labor induction included all methods for cervical ripening and induction with multiple values was allowed.
      Demographic data were summarized. We then investigated the precursors among women undergoing induction of labor at 24-41 weeks, which amounts to a weighted sample size of 1,323,407. One site did not provide indications for induction and was not included in the precursor analysis leaving a weighted sample size of 1,281,193.
      For the next analysis, the number and percentage of women with singleton gestations and vertex presentation (n = 1,231,662) who underwent induction of labor were calculated for each method of induction and stratified by both the need for cervical ripening and preterm (24-36 weeks of gestation) vs term (37-41 weeks of gestation). Kaplan-Meier curves were created for nulliparous women with a singleton gestation in vertex presentation and show the cervical dilation reached at the time of intrapartum cesarean delivery. These curves are stratified by the major precursor categories and by preterm (24 to <37 weeks) and term (37 to <42 weeks). Four hospitals did not report methods of induction, and 2 hospitals did not report cervical dilation at admission, leaving cases from 13 of the 19 hospitals available for analysis (n = 986,009). An additional 157,768 of the inductions did not have a method of induction reported and were excluded from the analysis. Of the remaining cases (n = 828,241), only 146,793 (17.7%) had an original Bishop score available with all 5 components reported, while 703,105 (84.9%) had information available on cervical dilation, effacement, and station. Therefore, as previously described elsewhere, we used a simplified Bishop score, comprised of dilation, effacement, and station only, to determine the cervical readiness (ripe vs unripe) for induction.
      • Laughon S.K.
      • Zhang J.
      • Troendle J.
      • Sun L.
      • Reddy U.M.
      Using a simplified Bishop score to predict vaginal delivery.
      We defined an unripe cervix as a simplified Bishop score ≤4 because of similar sensitivity and specificity to the original Bishop score ≤6, the definition of an unfavorable cervix.
      American College of Obstetricians and Gynecologists
      ACOG practice bulletin no. 107 Induction of labor.
      We compared maternal, obstetrical, and hospital characteristics between those with missing information and those where either the method of induction or cervical information was known. Women with a missing method of induction were slightly more likely to have a ripe cervix (21.8% vs 18.1%), but slightly more likely to deliver by cesarean (22.9% vs 20.1%). Cesarean delivery was also higher in women with a missing simplified Bishop score (25.8% vs 19.8%).
      Given that this is a descriptive analysis with very large sample size, no significance testing was performed. All analyses were performed using software (SAS, version 9.2; SAS Institute Inc, Cary, NC).

      Results

      The prevalence of induction of labor by maternal and obstetric characteristics is presented in Table 1. Overall, induction was common, occurring in 42.9% of nulliparous women and 31.8% of multiparous women. Among women attempting vaginal delivery, induction was more prevalent with increasing maternal age, those with higher BMI, and postdate pregnancies. The rate of induction varied modestly by race/ethnicity, ranging from 41.6% among non-Hispanic whites to 46.7% among Hispanic nulliparas, with multiracial/other/unknown appreciably lower rate (35.3%), and ranging in multiparas from 28.0% for multiracial/other/unknown to 33.8% among non-Hispanic whites. Induction in nulliparas varied substantially by hospital type, occurring most frequently in university-affiliated teaching hospitals (47.6%), followed by teaching community hospitals (36.7%), then nonteaching community hospitals (27.4). Rates by hospital type for multiparas were more similar (30.0-37.4%). The prevalence of induction in multiparous women with a previous uterine scar was low (8.3%). Induction was less common at gestational ages <34 weeks, and had the highest prevalence at 41 weeks of gestation (63.4% in nulliparas and 50.1% in multiparas).
      TABLE 1Prevalence of induction of labor (weighted) by parity
      NulliparousMultiparous
      VariableProportion of population, % n = 1,612,035Prevalence in women undergoing induction (42.9%), %Prevalence in women attempting vaginal delivery
      Denominator excludes prelabor cesarean delivery;
      (47.6%), %
      Proportion of population, % n = 2,033,140Prevalence in women undergoing induction (31.8%), %Prevalence in women attempting vaginal delivery
      Denominator excludes prelabor cesarean delivery;
      (41.0%), %
      Maternal age, y
       <186.537.238.90.423.426.3
       18-3483.043.347.477.831.940.0
       ≥3510.443.955.721.731.845.6
       Missing0 .122.634.50.114.220.3
      Race/ethnicity
       Non-Hispanic white58.741.646.654.333.843.7
       Non-Hispanic black13.644.148.114.629.137.9
       Hispanic20.646.750.825.029.738.0
       Asian/Pacific Islander3.844.448.82.930.438.6
       Other/unknown3.335.339.33.228.035.9
      Health insurance
       Private55.841.045.951.232.942.7
       Public31.139.743.234.827.436.0
       Self-pay1.247.151.01.628.935.4
       Other/unknown11.960.066.212.439.848.2
      BMI at delivery, kg/m2
       <25.012.938.042.110.329.335.3
       25.0-29.932.742.246.229.732.439.9
       30.0-34.921.646.551.823.133.744.2
       ≥35.015.053.561.018.535.151.3
       Unknown17.934.738.118.426.533.7
      No. of fetuses
       Singleton99.043.247.697.732.241.2
       Multiple1.018.136.12.315.631.6
      Previous uterine scar
      Includes previous myomectomy in nulliparous women;
      0.69.549.025.08.328.8
      Gestational age at delivery, wk
       <345.123.433.24.915.627.3
       34-368.241.649.99.525.636.8
       379.043.049.210.930.240.2
       3818.541.947.322.729.640.5
       3926.739.843.730.634.344.6
       4023.444.046.016.333.736.9
       ≥419.263.465.05.150.153.5
      Pregnancy complications
       Diabetes
      Pregestational or gestational;
      4.952.464.06.535.956.3
       Hypertensive disorder
      Chronic hypertension, preeclampsia, superimposed preeclampsia, eclampsia, gestational hypertension, unspecified;
      10.668.178.67.247.169.3
       Fetal anomaly
      For multiple gestations is per unit of pregnancy.
      7.937.447.47.626.640.1
      Hospital type
       University-affiliated teaching hospital60.047.653.660.532.543.1
       Teaching community hospital36.836.739.635.730.037.3
       Nonteaching community hospital3.227.429.23.837.444.9
      BMI, body mass index.
      Laughon. Induction of labor. Am J Obstet Gynecol 2012.
      a Denominator excludes prelabor cesarean delivery;
      b Includes previous myomectomy in nulliparous women;
      c Pregestational or gestational;
      d Chronic hypertension, preeclampsia, superimposed preeclampsia, eclampsia, gestational hypertension, unspecified;
      e For multiple gestations is per unit of pregnancy.

      Precursors of induction

      Indicated precursors of induction were the largest category of precursors in singleton gestations, regardless of gestational age or parity (Table 2). The most common indicated precursor for preterm induction in singleton gestations was hypertensive disease (42.6% in nulliparas and 31.6% in multiparas), followed by fetal (28.7% in nulliparas and 27.3% in multiparas) and maternal (24.7% and 27.1%, respectively) conditions. At term, the prevalence of indicated precursors was reversed, with fetal (20.6% in nulliparas and 12.8% in multiparas) and maternal (14.8% in nulliparas and 16.6% in multiparas) conditions more common than hypertensive disease (14.2% in nulliparas and 8.7% in multiparas). Inductions were elective in 1.2% of preterm nulliparas and 2.3% of preterm multiparas, compared to 15.6% of nulliparas and 25.4% multiparas at term. However, 9.9% of all preterm inductions were classified as elective or no recorded indication. Approximately 95% of pregnancies with the unknown precursors for induction delivered at ≥37 weeks of gestation; 4.3% were at late preterm; and 0.9% were <34 weeks of gestation. Elective induction was the most common individual precursor in multiparas at term.
      TABLE 2Precursors for induction of labor by gestational age with rate of vaginal delivery in singleton gestations
      Preterm 24-36 wkTerm 37-41 wk
      PrecursorNulliparous n = 69,796Multiparous n = 58,432Nulliparous n = 595,585Multiparous n = 557,380
      With precursor, %
      Any precursor
      Categories for “indicated” precursors can add up to more than total indicated percentage because women could have >1 pregnancy condition;
       PROM22.322.010.56.3
       Chorioamnionitis2.62.31.20.3
       Decidual hemorrhage/abruption3.75.41.51.9
       Hypertensive disease42.631.614.28.7
        Gestational hypertension13.36.94.22.0
        Preeclampsia14.17.44.52.0
        Superimposed preeclampsia6.36.31.10.9
        Eclampsia1.20.40.030.1
        Chronic hypertension2.32.41.11.3
        Unspecified5.38.33.42.5
       Maternal condition
      Maternal medical problems–percent of women with diabetes is listed;
      24.727.114.816.6
        Diabetes11.111.95.67.1
       Fetal anomaly15.611.36.05.7
       Antepartum stillbirth4.65.60.20.7
       Suspected fetal macrosomia0.60.92.12.4
       Fetal condition
      Included conditions such as intrauterine growth restriction and abnormal antenatal testing;
      28.727.320.612.8
       Maternal fever on admission1.50.40.60.2
       Admission for fetal reason, not otherwise specified
      Were included only if there was no other pregnancy condition–these are the only 2 indicated categories that are exclusive of other indications;
      0.20.60.60.6
       Admission for maternal reason, not otherwise specified
      Were included only if there was no other pregnancy condition–these are the only 2 indicated categories that are exclusive of other indications;
      1.41.10.60.6
       History of maternal/obstetrical condition
      Included pregnancy complications in prior pregnancy (eg, traumatic first delivery or history of fetal demise).
      0.030.40.020.6
       History of fetal condition
      Included pregnancy complications in prior pregnancy (eg, traumatic first delivery or history of fetal demise).
      0.68.00.36.3
       Postdates0011.46.4
       Prior uterine scar0.27.60.16.3
      Total “indicated” | Vaginal delivery, %91.2 | 62.888.9 | 80.364.5 | 63.755.9 | 85.5
      Elective | Vaginal delivery, %1.2 | 92.32.3 | 95.815.6 | 76.225.4 | 97.0
      No recorded indication | Vaginal delivery, %7.6 | 83.48.8 | 93.419.9 | 75.918.7 | 94.6
      One site did not provide indications for induction and was excluded.
      PROM, premature rupture of membranes.
      Laughon. Induction of labor. Am J Obstet Gynecol 2012.
      a Categories for “indicated” precursors can add up to more than total indicated percentage because women could have >1 pregnancy condition;
      b Maternal medical problems–percent of women with diabetes is listed;
      c Included conditions such as intrauterine growth restriction and abnormal antenatal testing;
      d Were included only if there was no other pregnancy condition–these are the only 2 indicated categories that are exclusive of other indications;
      e Included pregnancy complications in prior pregnancy (eg, traumatic first delivery or history of fetal demise).

      Success of vaginal delivery by precursor for induction

      Results for delivery outcomes are also presented in Table 2. Among nulliparous women, indicated induction was associated with the lowest vaginal delivery rates (62.8% preterm and 63.7% term). At term, elective induction in multiparas was associated with a high vaginal delivery rate of 97% vs 76.2% for nulliparas. Vaginal delivery rates for elective induction were similar to those with no recorded indication. Among nulliparous women, both preterm and term, the proportions still in labor at each cervical dilation were highest for elective induction, those with no recorded indication, and ROM (Figure). With respect to precursors of preterm delivery, the largest percentage of cesarean deliveries in the first stage of labor were performed <6-cm dilation, especially for fetal indications and preeclampsia, followed by diabetes (Figure, A). The rate of cesarean delivery >6 cm decreased modestly in active labor. At term, the largest percentages of cesarean deliveries in the first stage of labor were also performed <6-cm dilation, and occurred more often for fetal indications, diabetes, postdates, and preeclampsia compared to those with elective induction, no recorded indication, or ROM.
      Figure thumbnail gr1
      FIGURECervical dilation at cesarean delivery
      A, Preterm, 24 to <37 and B, term, 37-41 weeks of gestation. Cervical dilation at intrapartum cesarean delivery among nulliparous women with singleton gestations undergoing induction of labor by precursor category.
      ROM, rupture of membranes.
      Laughon. Induction of labor. Am J Obstet Gynecol 2012.

      Method of induction and success of vaginal delivery

      Oxytocin was the most common method of induction, regardless of gestational age, parity, or cervical ripeness (TABLE 3, TABLE 4). Artificial ROM was the second-most common method of induction for all categories. Overall, misoprostol and prostaglandin E2 were used more commonly than mechanical methods for an unripe cervix, whereas mechanical method use was similar at term compared to preterm in nulliparas (5.1% vs 6.2%), but not in multiparas (2.9% vs 4.9%). Regardless of method, vaginal delivery rates were high for multiparas with a ripe cervix both preterm (94.6-100.0%) and term (86.6-100.0%). Both nulliparas and multiparas with an unripe cervix preterm and at term had lower vaginal delivery rates, compared to those cases with a ripe cervix, for each method of induction.
      TABLE 3Method of induction by cervical status in preterm singleton gestations with rate of vaginal delivery
      Preterm 24-36 wk Method, % (vaginal delivery, %)
      Numbers for method of induction are percent of women by parity and cervical ripeness who underwent that method (row adds up to 100%). Numbers for vaginal delivery are percent of women who had vaginal delivery who underwent induction per method by parity and cervical ripeness. For example, of 6.3% nulliparous women with unripe cervix who were induced with misoprostol, 60.7% had vaginal delivery.
      NulliparousMultiparous
      Unripe n = 30,574Ripe n = 4900Unripe n = 25,167Ripe n = 3662
      VariableWith methodVaginal deliveryWith methodVaginal deliveryWith methodVaginal deliveryWith methodVaginal delivery
      Misoprostol6.360.74.393.34.383.91.7100.0
      PGE28.238.63.177.06.665.20.7100.0
      Misoprostol and PGE20.778.200.553.30
      Mechanical1.138.30.9100.00.970.50.4100.0
      Mechanical and misoprostol or PGE21.563.30.5100.01.290.00.6100.0
      Artificial ROM11.274.16.689.512.189.519.294.6
      Oxytocin71.062.884.690.974.485.377.696.2
      Data are for singleton gestations with vertex presentation. Unripe cervix defined as simplified Bishop score (dilation, effacement, and station only) ≤4. All methods could include oxytocin. Oxytocin category is exclusive (only oxytocin). Data are weighted.
      PGE2, prostaglandin E2; ROM, rupture of membranes.
      Laughon. Induction of labor. Am J Obstet Gynecol 2012.
      a Numbers for method of induction are percent of women by parity and cervical ripeness who underwent that method (row adds up to 100%). Numbers for vaginal delivery are percent of women who had vaginal delivery who underwent induction per method by parity and cervical ripeness. For example, of 6.3% nulliparous women with unripe cervix who were induced with misoprostol, 60.7% had vaginal delivery.
      TABLE 4Method of induction by cervical status in term singleton gestations with rate of vaginal delivery
      Term 37-41 wk Method, % (vaginal delivery, %)
      Numbers for method of induction are percent of women by parity and cervical ripeness who underwent that method (row adds up to 100%). Numbers for vaginal delivery are percent of women who had vaginal delivery who underwent induction per method by parity and cervical ripeness. For example, of 6.3% nulliparous women with unripe cervix who were induced with misoprostol, 60.7% had vaginal delivery.
      NulliparousMultiparous
      Unripe n = 252,638Ripe n = 79,326Unripe n = 235,470Ripe n = 71,368
      VariableWith methodVaginal deliveryWith methodVaginal deliveryWith methodVaginal deliveryWith methodVaginal delivery
      Misoprostol2.556.81.373.92.583.70.798.6
      PGE25.558.04.389.25.587.74.298.6
      Misoprostol and PGE20.457.90.1100.00.485.70.186.6
      Mechanical0.357.00.2100.00.370.30.1100.0
      Mechanical and misoprostol or PGE21.571.11.691.81.594.91.297.2
      Artificial ROM25.274.523.088.625.295.037.198.6
      Oxytocin64.667.869.583.364.691.756.797.0
      Data are for singleton gestations with vertex presentation. Unripe cervix defined as simplified Bishop score (dilation, effacement, and station only) ≤4. All methods could include oxytocin. Oxytocin category is exclusive (only oxytocin). Data are weighted.
      PGE2, prostaglandin E2; ROM, rupture of membranes.
      Laughon. Induction of labor. Am J Obstet Gynecol 2012.
      a Numbers for method of induction are percent of women by parity and cervical ripeness who underwent that method (row adds up to 100%). Numbers for vaginal delivery are percent of women who had vaginal delivery who underwent induction per method by parity and cervical ripeness. For example, of 6.3% nulliparous women with unripe cervix who were induced with misoprostol, 60.7% had vaginal delivery.

      Comment

      This study describes maternal and obstetric characteristics of induction of labor in a large, modern cohort of parturients across the United States. Induction of labor was a common obstetric intervention, occurring in 47.6% of nulliparous and 41.0% of multiparous women attempting vaginal delivery. Induction was more prevalent among women who were older and with higher BMI, but did not vary much across race/ethnicity. Hypertensive disease, maternal conditions including diabetes, and fetal conditions were the most common indicated precursors for induction. While precursors for induction differed by gestational age and parity, approximately 90% of preterm inductions were for medical and obstetrical complications, whereas elective induction and induction with no recorded indication occurred in about one-third of nulliparas and almost half of multiparas at term.
      Successful vaginal delivery rates varied by precursors and also depended on gestational age. For all precursors, cesarean delivery in the first stage of labor was more common in nulliparas <6 cm. In a previous study of women who presented in spontaneous labor from this same original cohort, active labor did not start until 6-cm dilation.
      • Zhang J.
      • Landy H.J.
      • Branch D.W.
      • et al.
      Contemporary patterns of spontaneous labor with normal neonatal outcomes.
      This finding is not surprising since there is no agreed upon definition of “failed labor induction.”
      • Rouse D.J.
      • Weiner S.J.
      • Bloom S.L.
      • et al.
      Failed labor induction: toward an objective diagnosis.
      Indicated precursors for induction, including preeclampsia, diabetes, and fetal indications, were associated with higher cesarean delivery rates, compared to women undergoing elective induction or with ROM or no recorded indication, at every centimeter of dilation <6 cm of dilation, indicating that inductions were more likely to fail during the latent phase in complicated pregnancies. This result may be due to a combination of worsening maternal status and/or fetal intolerance of labor, as well as perceived need for a more expedited delivery. It is worth noting that among nulliparous women undergoing induction at term for elective reasons or without a recorded indication, suggestive of healthy pregnancies, 16.0% and 12.3%, respectively, underwent cesarean delivery in the latent phase of labor.
      Oxytocin alone was the most common method of induction, which is consistent with previous studies.
      • Alfirevic Z.
      • Kelly A.J.
      • Dowswell T.
      Intravenous oxytocin alone for cervical ripening and induction of labor.
      Vaginal delivery rates were higher for women with a ripe vs an unripe cervix for all methods of induction. A Cochrane Review found prostaglandins to be associated with higher successful vaginal delivery within 24 hours, as compared to oxytocin; however, most studies did not account for cervical ripeness.
      • Alfirevic Z.
      • Kelly A.J.
      • Dowswell T.
      Intravenous oxytocin alone for cervical ripening and induction of labor.
      Mechanical methods were associated with higher vaginal delivery rates, compared to oxytocin alone, which is consistent with the literature.
      • Boulvain M.
      • Kelly A.
      • Lohse C.
      • Stan C.
      • Irion O.
      Mechanical methods for induction of labor.
      Prostaglandins and mechanical cervical ripening were associated with similar vaginal delivery rates in nulliparas, whereas the existing evidence has been insufficient to determine whether prostaglandins or mechanical cervical ripening is more effective.
      • Boulvain M.
      • Kelly A.
      • Lohse C.
      • Stan C.
      • Irion O.
      Mechanical methods for induction of labor.
      At term, mechanical ripening together with prostaglandin (either misoprostol and/or prostaglandin E2) was generally associated with a higher successful vaginal delivery rate, compared to other methods. It may be that the combination of multiple methods of induction is superior. Yet this result could also reflect the clinician's commitment to a successful induction, given the willingness to try multiple methods prior to proceeding with cesarean delivery. We did not have information, however, on whether these methods were used simultaneously or in succession.
      Our study is limited because the indications for induction were not always provided. We supplemented indications with information on any other medical, obstetrical, or fetal conditions, even though those may not have been the actual indication for delivery. Therefore, the true incidence of indicated precursors was likely less than we have reported. Some of the cases with no recorded indication may be due to underreporting of maternal or fetal conditions. At the same time, given the large number of variables on which data were collected, as well as our conservative effort to include all possible conditions, a certain proportion of the deliveries with an unknown precursor were likely elective. Also, the fact that the majority of the precursors with no indication were at term and associated with the highly successful vaginal delivery rate similar to the rate observed for elective indication provides further support that those with no recorded indication were more likely to be cases of elective induction. Our study is further limited by instances of missing methods of induction, although it is likely that they are missing at random since the percentage of indicated precursors were the same whether the method of induction was recorded or not. It was also likely that the recording of some vs all 3 components of the simplified Bishop score was based on clinicians' preferences, rather than inherent differences about women undergoing induction. While the results may not be applicable to the entire US population, the major strengths of our study are the large sample size and the ability to provide a comprehensive description of induction of labor in a modern obstetrical cohort across the United States.
      In conclusion, we found that induction of labor in a modern obstetric cohort occurred in almost half of nulliparous women and 4 of 10 multiparous women attempting vaginal delivery. Precursors differed by gestational age and parity, and vaginal delivery was more successful in uncomplicated pregnancies. Approximately 1 of 10 preterm inductions did not have an indication for induction or another pregnancy condition, despite the evidence that neonatal morbidity and mortality decreases with each gestational week until 39 weeks of gestation.
      • Laughon S.K.
      • Reddy U.M.
      • Sun L.
      • Zhang J.
      Precursors for late preterm birth in singleton gestations.
      • Reddy U.M.
      • Bettegowda V.R.
      • Dias T.
      • Yamada-Kushnir T.
      • Ko C.W.
      • Willinger M.
      Term pregnancy: a period of heterogeneous risk for infant mortality.
      Elective induction and those with no recorded indication were common at term, comprising almost one third of nulliparous and almost half of multiparous inductions, with a high rate of successful vaginal delivery, in multiparas regardless of Bishop score. Considering the findings that nulliparous women at term with induction for elective reasons or with no recorded indication had an associated 24% rate of cesarean delivery with the majority being performed in latent labor, selecting appropriate candidates; avoiding elective induction in nulliparas, especially with an unripe cervix; and waiting longer for labor to progress into the active phase (>6 cm) would make an impact on decreasing the national cesarean delivery rate.

      Acknowledgments

      Institutions involved in the Consortium include, in alphabetical order: Baystate Medical Center, Springfield, MA; Cedars-Sinai Medical Center Burnes Allen Research Center, Los Angeles, CA; Christiana Care Health System, Newark, DE; The EMMES Corporation, Rockville, MD (Data Coordinating Center); Georgetown University Hospital, MedStar Health, Washington, DC; Indiana University Clarian Health, Indianapolis, IN; Intermountain Healthcare and the University of Utah, Salt Lake City, UT; Maimonides Medical Center, Brooklyn, NY; MetroHealth Medical Center, Cleveland, OH; Summa Health System, Akron City Hospital, Akron, OH; University of Illinois at Chicago, Chicago, IL; University of Miami, Miami, FL; and University of Texas Health Science Center at Houston, Houston, TX.

      References

        • Martin J.A.
        • Hamilton B.E.
        • Sutton P.D.
        • Ventura S.J.
        • Menacker F.
        • Kirmeyer S.
        Births: final data for 2006.
        National Vital Statistics Report. 2009; 57: 7
        • Zhang J.
        • Yancey M.K.
        • Henderson C.E.
        US national trends in labor induction, 1989-1998.
        J Reprod Med. 2002; 47: 120-124
        • Cleary-Goldman J.
        • Malone F.D.
        • Vidaver J.
        • et al.
        Impact of maternal age on obstetric outcome.
        Obstet Gynecol. 2005; 105: 983-990
        • Villamor E.
        • Cnattingius S.
        Interpregnancy weight change and risk of adverse pregnancy outcomes: a population-based study.
        Lancet. 2006; 368: 1164-1170
        • Lydon-Rochelle M.T.
        • Cardenas V.
        • Nelson J.C.
        • Holt V.L.
        • Gardella C.
        • Easterling T.R.
        Induction of labor in the absence of standard medical indications: incidence and correlates.
        Med Care. 2007; 45: 505-512
        • Ehrenthal D.B.
        • Jiang X.
        • Strobino D.M.
        Labor induction and the risk of a cesarean delivery among nulliparous women at term.
        Obstet Gynecol. 2010; 116: 35-42
        • Caughey A.B.
        • Sundaram V.
        • Kaimal A.J.
        • et al.
        Systematic review: elective induction of labor versus expectant management of pregnancy.
        Ann Intern Med. 2009; 151 (W53-63): 252-263
        • Zhang J.
        • Troendle J.
        • Reddy U.M.
        • et al.
        Contemporary cesarean delivery practice in the United States.
        Am J Obstet Gynecol. 2010; 203: 326.e1-326.e10
        • Martin J.A.
        • Hamilton B.E.
        • Sutton P.D.
        • Ventura S.J.
        • Menacker F.
        • Kirmeyer S.
        Births: final data for 2004.
        Natl Vital Stat Rep. 2006; 55: 1-101
        • Laughon S.K.
        • Zhang J.
        • Troendle J.
        • Sun L.
        • Reddy U.M.
        Using a simplified Bishop score to predict vaginal delivery.
        Obstet Gynecol. 2011; 117: 805-811
        • American College of Obstetricians and Gynecologists
        ACOG practice bulletin no. 107.
        Obstet Gynecol. 2009; 114: 386-397
        • Zhang J.
        • Landy H.J.
        • Branch D.W.
        • et al.
        Contemporary patterns of spontaneous labor with normal neonatal outcomes.
        Obstet Gynecol. 2010; 116: 1281-1287
        • Rouse D.J.
        • Weiner S.J.
        • Bloom S.L.
        • et al.
        Failed labor induction: toward an objective diagnosis.
        Obstet Gynecol. 2011; 117: 267-272
        • Alfirevic Z.
        • Kelly A.J.
        • Dowswell T.
        Intravenous oxytocin alone for cervical ripening and induction of labor.
        Cochrane Database Syst Rev. 2009; 4 (CD003246)
        • Boulvain M.
        • Kelly A.
        • Lohse C.
        • Stan C.
        • Irion O.
        Mechanical methods for induction of labor.
        Cochrane Database Syst Rev. 2001; 4 (CD001233)
        • Laughon S.K.
        • Reddy U.M.
        • Sun L.
        • Zhang J.
        Precursors for late preterm birth in singleton gestations.
        Obstet Gynecol. 2010; 116: 1047-1055
        • Reddy U.M.
        • Bettegowda V.R.
        • Dias T.
        • Yamada-Kushnir T.
        • Ko C.W.
        • Willinger M.
        Term pregnancy: a period of heterogeneous risk for infant mortality.
        Obstet Gynecol. 2011; 117: 1279-1287