A 34-year-old woman, gravida 5 para 4, at 39 weeks gestation is admitted to labor and delivery for contractions and spontaneous rupture of membranes. Her pregnancy has been uncomplicated, and her prior pregnancies ended in term vaginal deliveries. Prepregnancy BMI was 33 kg/m² and she gained 18 kg (40 lb) this pregnancy. Two hours after admission, the patient receives neuraxial anesthesia and no longer feels contraction pain. After placement of neuraxial anesthesia, she has recurrent variable decelerations so an intrauterine catheter is placed. The variable decelerations resolve with amnioinfusion, and her labor continues. The patient's cervical examinations are shown on the labor curve below: The fetal head is in the occiput posterior position. Fetal heart rate monitoring is category 1. An intrauterine pressure catheter shows 200 Montevideo units over 10 minutes for the past 4 hours. Which of the following is the most likely cause of this patient's labor curve?

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11ec25c8_da47_d3d8_bc81_c142f47bdb7f_MD0003_00 The first stage of labor (from start of contractions to 10 cm cervical dilation) includes a latent and an active phase. The active phase of labor (6-10 cm cervical dilation) has an expected, predictable rate of cervical dilation of ≥1 cm every 2 hours. This patient reached the active phase (6 cm cervical dilation) when she received her epidural 2 hours after admission. She then progressed appropriately from 6 cm to 7 cm cervical dilation over the next 2 hours (cervical dilation ≥1 cm/2 hr). However, her cervix dilated only 1 cm between hours 4 and 8 (<1 cm every 2 hours), consistent with active phase protraction. Protraction is commonly caused by cephalopelvic disproportion, in which the fetal head is too large to fit through the maternal pelvis. Cephalopelvic disproportion is more common in late-term pregnancies (≥41 weeks gestation) or in cases of fetal anomaly or malposition (eg, occiput posterior). Additional risk factors include maternal obesity, excessive weight gain, nulliparity, advanced maternal age, and inadequate contractions. (Choice B) Hypocontractile uterine activity (ie, inadequate contractions) can lead to labor protraction. However, this patient's contractions are adequate (≥200 Montevideo units), as measured by the intrauterine pressure catheter. Increasing their strength above 200 Montevideo units would not change the labor curve. (Choice C) Fetal gestational age ≥41 weeks gestation (late-term pregnancy) can be a risk factor for protracted or arrested labor due to larger fetal size. This patient is at 39 weeks gestation. (Choice D) Nulliparous patients (primigravidas) experience slower first and second stages of labor compared to parous patients (multigravidas). This patient has had 4 prior term vaginal deliveries, which would predict an accelerated, not protracted, labor curve. (Choice E) Neuraxial anesthesia (eg, epidural) can lengthen the second stage of labor (10 cm cervical dilation until fetal delivery) but not the first stage. This patient is in the first stage of labor. Also, her labor became protracted (<1 cm/2 hr) starting at hour 4, independent of her epidural placement at hour 2. Educational objective: Active phase protraction is <1 cm cervical dilation in 2 hours during the active phase of labor (6-10 cm cervical dilation). Cephalopelvic disproportion, in which the fetal head is too large to fit through the maternal pelvis, is a common cause of labor protraction. Other risk factors include inadequate contractions, maternal obesity, and fetal malposition (eg, occiput posterior). References: Contemporary patterns of spontaneous labor with normal neonatal outcomes. (http://www.ncbi.nlm.nih.gov/pubmed/21099592) Preventing the first cesarean delivery: summary of a joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, and American College of Obstetricians and Gynecologists workshop. (http://www.ncbi.nlm.nih.gov/pubmed/23090537)

A 34-Year-Old Woman, Gravida 5 Para 4, at 39 Weeks Gestation