A 33-year-old woman, gravida 2 para 1, at 40 weeks gestation is admitted to the hospital due to contractions and spontaneous rupture of membranes. The patient underwent a cesarean delivery with her first child due to breech presentation; this pregnancy has been uncomplicated. She has no chronic medical conditions and is taking only a prenatal vitamin. Her pre-pregnancy BMI was 20 kg/m² and she has gained 15.9 kg (35 lb) during the pregnancy. Blood pressure is 130/80 mm Hg. The patient is admitted, epidural anesthesia is administered, and an intrauterine pressure catheter is placed. She quickly dilates to 10 cm with the fetal vertex at 0 station, occiput transverse. Four hours later, the pelvic examination is unchanged but there is molding and caput on the fetal head. Fetal monitoring is category I. Contractions occur every 2-3 minutes and the patient pushes with each contraction. The contraction strength is an average of 210 Montevideo units every 10 minutes. Which of the following is the most likely etiology for this patient's clinical presentation?

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11ec25c8_da45_62d1_bc81_b3d167c0dcac_MD0003_00 The second stage of labor begins when the cervix is 10 cm dilated and ends with fetal delivery. The duration of the second stage is affected by parity and use of neuraxial anesthesia. Progression during the second stage is evaluated by determining the fetal station, which measures the descent of the fetal head through the pelvis. fetal station An arrested second stage occurs when there is no fetal descent after pushing for ≥3 hours in nulliparous patients (≥2 hours in multiparous patients). This patient is functionally nulliparous because her first child was a cesarean delivery for breech presentation. A common cause of a protracted or arrested second stage is fetal malposition. Fetal position is the relationship of the fetal presenting part to the maternal pelvis. The optimal fetal position is occiput anterior because it facilitates the cardinal movements of labor. Deviations from this position (eg, occiput transverse) can cause cephalopelvic disproportion and second stage arrest. occiput anterior occiput transverse (Choice A) Inadequate contractions (<200 Montevideo units averaged over 10 minutes) commonly lead to labor arrest. This patient's contractions are adequate. (Choice C) Maternal obesity and excessive weight gain during pregnancy are risk factors for second stage arrest. This patient had a normal pre-pregnancy BMI (20 kg/m²) and has had appropriate weight gain (eg, 15.9 kg [35 lb]). ² (Choice D) Molding is the change in fetal skull shape as maternal expulsive efforts sculpt the fetal head into the shape of the pelvis to facilitate delivery. Caput is scalp edema due to prolonged pressure. Molding and caput suggest cephalopelvic disproportion, not poor maternal effort. (Choice E) Patients with a prior uterine myomectomy or cesarean delivery are at higher risk for uterine rupture. Patients often present with fetal heart rate abnormalities and a sudden loss of fetal station (eg, going from +1 to −3 station) because rupture and subsequent decreased intrauterine pressure causes fetal retreat upward and into the abdominal cavity through the uterine scar. This patient's fetal heart rate tracing is category 1 and fetal station has remained 0. Educational objective: Second stage arrest of labor occurs when there is insufficient fetal descent after pushing ≥3 hours in nulliparous patients (≥2 hours in multiparous patients). Fetal malposition (eg, nonocciput anterior) contributes to cephalopelvic disproportion, the most common cause of second stage arrest. References: Diagnosis and management of arrest disorders: duration to wait. (http://www.ncbi.nlm.nih.gov/pubmed/23009971) Weight gain during pregnancy: reexamining the guidelines. (http://www.ncbi.nlm.nih.gov/pubmed/20669500)

A 33-Year-Old Woman, Gravida 2 Para 1, at 40 Weeks