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Oxytocin utilization for women undergoing an induction of labor by 1 kg/m2 increase in body mass index

      Objective

      Women with obesity have higher oxytocin utilization and are simultaneously more likely to undergo and fail an induction of labor than women without obesity.
      American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins–Obstetrics
      Obesity in pregnancy: ACOG Practice Bulletin, Number 230.
      • Norman S.M.
      • Tuuli M.G.
      • Odibo A.O.
      • Caughey A.B.
      • Roehl K.A.
      • Cahill A.G.
      The effects of obesity on the first stage of labor.
      • Nuthalapaty F.S.
      • Rouse D.J.
      • Owen J.
      The association of maternal weight with cesarean risk, labor duration, and cervical dilation rate during labor induction.
      • Pevzner L.
      • Powers B.L.
      • Rayburn W.F.
      • Rumney P.
      • Wing D.A.
      Effects of maternal obesity on duration and outcomes of prostaglandin cervical ripening and labor induction.
      • Roloff K.
      • Peng S.
      • Sanchez-Ramos L.
      • Valenzuela G.J.
      Cumulative oxytocin dose during induction of labor according to maternal body mass index.
      Historically, research has focused on obesity as a categorical variable
      • Norman S.M.
      • Tuuli M.G.
      • Odibo A.O.
      • Caughey A.B.
      • Roehl K.A.
      • Cahill A.G.
      The effects of obesity on the first stage of labor.
      ,
      • Pevzner L.
      • Powers B.L.
      • Rayburn W.F.
      • Rumney P.
      • Wing D.A.
      Effects of maternal obesity on duration and outcomes of prostaglandin cervical ripening and labor induction.
      ,
      • Roloff K.
      • Peng S.
      • Sanchez-Ramos L.
      • Valenzuela G.J.
      Cumulative oxytocin dose during induction of labor according to maternal body mass index.
      despite the involvement of a heterogenous patient population, which may have important ramifications for induction of labor protocols such as using low- vs high-dose oxytocin.
      American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins–Obstetrics
      Obesity in pregnancy: ACOG Practice Bulletin, Number 230.
      In this study, we examined body mass index (BMI) at the time of delivery as a continuous variable by 1 kg/m2 increase and cumulative oxytocin utilization, average oxytocin rate, maximum oxytocin rate, length of induction, and cesarean delivery.

      Study Design

      Data were collected retrospectively on women with term, singleton pregnancies undergoing an induction of labor from January 1, 2018 through June 20, 2018 at a large academic center. Women with pregnancies complicated by fetal anomalies or stillbirth (n=8), previous cesarean (n=33), lack of oxytocin utilization (n=252), or missing BMI data (n=4) were excluded. The primary independent variable was BMI at the time of delivery, and the primary outcome was cumulative oxytocin utilization. A secondary effect measure modifier was parity, and the secondary outcomes included average oxytocin rate, maximum oxytocin rate, length of induction, and cesarean delivery. The labor, maternal, and neonatal characteristics were collected by the medical record. The adjusted mean difference or odds ratios were calculated using multivariable linear and logistic regression.

      Results

      Of the 797 women in our cohort, 531 (66.6%) had obesity with a mean BMI of 34 kg/m2. Women with obesity were more likely to have hypertensive disorders of pregnancy (112 vs 33; P=.003), less likely to have pregnancies complicated by fetal growth restriction (23 vs 35; P<.001), more likely to deliver a neonate with a higher mean birthweight (3380 vs 3175 g; P<.001), and more likely to have a cesarean delivery but no difference for indication. The most common indication was arrest of dilation/descent. The mean oxytocin rate was 6.21 mU/min, mean maximum rate of oxytocin was 10.3 mU/min, and mean length of induction was approximately 10 hours and 58 minutes in our cohort. There were BMI-dependent associations per 1 kg/m2 difference in BMI for approximately all outcomes (Table). Regardless of parity, the cumulative oxytocin utilization was significantly higher per 1 kg/m2 increase in BMI after adjusting for confounders (adjusted mean difference [95% confidence interval], 141.1 [38.2–244.0] and 139.8 [45.3–234.4] units for nulliparous and multiparous women, respectively). The length of induction was also increased by 17.3 (9.4–25.2) and 9.4 (2.4–16.4) minutes for nulliparous and multiparous women, respectively, per 1 kg/m2 increase in BMI. In other words, a nulliparous woman with a BMI of 40 kg/m2 would require an approximately 4 hours longer induction than a nulliparous woman with a BMI of 25 kg/m2. These associations persisted when the analytical population included only those women who had a successful vaginal delivery.
      TableAssociation of outcomes per body mass index (in kg/m2) unit at time of delivery among women undergoing an induction of labor
      Outcomes by parityAdjusted mean difference
      Mean difference (slope, beta) per unit increase in BMI. Adjusted for magnesium, fetal growth restriction, and hypertensive disorders by multivariable linear regression


      (95% CI)
      Adjusted odds ratio
      Odds ratio for cesarean delivery per unit increase in BMI. Adjusted for magnesium, fetal growth restriction, and hypertensive disorders by multivariable logistic regression.


      (95% CI)
      Nulliparous (n=460)
       Cumulative oxytocin dose, units141.1 (38.2–244.0)
       Average oxytocin rate, mU/min0.04 (−0.02 to 0.09)
       Maximum oxytocin rate, mU/min0.13 (0.04–0.22)
       Length of induction (min)17.3 (9.4–25.2)
       Cesarean delivery1.07 (1.03–1.10)
      Multiparous (n=317)
       Cumulative oxytocin dose, milliunits139.8 (45.3–234.4)
       Average oxytocin rate, mU/min0.10 (0.03–0.17)
       Maximum oxytocin rate, mU/min0.19 (0.08–0.30)
       Length of induction, min9.4 (2.4–16.4)
       Cesarean delivery1.05 (0.99–1.12)
      BMI defined as weight/height2 at the time of admission for delivery; induction defined as nonspontaneous labor; length of induction defined from start of oxytocin.
      BMI, body mass index; CI, confidence interval.
      Polnaszek. Oxytocin utilization for women undergoing an induction of labor by 1 kg/m2 increase in body mass index. Am J Obstet Gynecol 2022.
      a Mean difference (slope, beta) per unit increase in BMI. Adjusted for magnesium, fetal growth restriction, and hypertensive disorders by multivariable linear regression
      b Odds ratio for cesarean delivery per unit increase in BMI. Adjusted for magnesium, fetal growth restriction, and hypertensive disorders by multivariable logistic regression.

      Conclusion

      Oxytocin utilization in women undergoing an induction of labor demonstrates BMI-dependent association at the time of delivery. Future studies should focus on BMI-dependent induction protocols for oxytocin utilization.

      References

        • American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins–Obstetrics
        Obesity in pregnancy: ACOG Practice Bulletin, Number 230.
        Obstet Gynecol. 2021; 137: e128-e144
        • Norman S.M.
        • Tuuli M.G.
        • Odibo A.O.
        • Caughey A.B.
        • Roehl K.A.
        • Cahill A.G.
        The effects of obesity on the first stage of labor.
        Obstet Gynecol. 2012; 120: 130-135
        • Nuthalapaty F.S.
        • Rouse D.J.
        • Owen J.
        The association of maternal weight with cesarean risk, labor duration, and cervical dilation rate during labor induction.
        Obstet Gynecol. 2004; 103: 452-456
        • Pevzner L.
        • Powers B.L.
        • Rayburn W.F.
        • Rumney P.
        • Wing D.A.
        Effects of maternal obesity on duration and outcomes of prostaglandin cervical ripening and labor induction.
        Obstet Gynecol. 2009; 114: 1315-1321
        • Roloff K.
        • Peng S.
        • Sanchez-Ramos L.
        • Valenzuela G.J.
        Cumulative oxytocin dose during induction of labor according to maternal body mass index.
        Int J Gynaecol Obstet. 2015; 131: 54-58