CASE 7- to 8-lb baby suspected to be in occiput posterior (OP) position
A certified nurse midwife (CNM) asks you to consult on a 37-year-old woman (G1P0) at 41 weeks’ gestation who was admitted to labor and delivery for a late-term induction. The patient had a normal first stage of labor with placement of a combined spinal-epidural anesthetic at a cervical dilation of 4 cm. She has been fully dilated for 3.5 hours and pushing for 2.5 hours with a Category 1 fetal heart rate tracing. The CNM reports that the estimated fetal weight is 7 to 8 lb and the station is +3/5. She suspects that the fetus is in the left OP position. She asks for your advice on how to best deliver the fetus. The patient strongly prefers not to have a cesarean delivery (CD).
What is your recommended approach?
The cardinal movements of labor include cephalic engagement, descent, flexion, internal rotation, extension and rotation of the head at delivery, internal rotation of the shoulders, and expulsion of the body. In the first stage of labor many fetuses are in the OP position. Flexion and internal rotation of the fetal head in a mother with a gynecoid pelvis results in most fetuses assuming an occiput anterior (OA) position with the presenting diameter of the head (occipitobregmatic) being optimal for spontaneous vaginal delivery. Late in the second stage of labor only about 5% of fetuses are in the OP position with the presenting diameter of the head being large (occipitofrontal) with an extended head attitude, thereby reducing the probability of a rapid spontaneous vaginal delivery.
Risk factors for OP position late in the second stage of labor include1,2:
- body mass index > 29 kg/m2
- gestation age ≥ 41 weeks
- birth weight > 4 kg
- regional anesthesia.
Maternal outcomes associated with persistent OP position include protracted first and second stage of labor, arrest of second stage of labor, and increased rates of operative vaginal delivery, anal sphincter injury, CD, postpartum hemorrhage, chorioamnionitis, and endomyometritis.1,3,4 The neonatal complications of persistent OP position include increased rates of shoulder dystocia, low Apgar score, umbilical artery acidemia, meconium, and admission to a neonatal intensive care unit.1,5
Many obstetricians report that they can reliably detect a fetus in the OP position based upon abdominal palpation of the fetal spine and digital vaginal examination of the fetal sutures, fontanels, and ears. Such self-confidence may not be wholly warranted, however. Most contemporary data indicate that digital vaginal examination has an error rate of approximately 20% for identifying the position of the cephalic fetus, especially in the presence of fetal caput succedaneum and asynclitism.6-10
The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that cephalic position be determined by transabdominal imaging.11 By placing the ultrasound probe on the maternal abdomen, a view of the fetal body at the level of the chest helps determine the position of the fetal spine. When the probe is placed in a suprapubic position, the observation of the fetal orbits facing the probe indicates an OP position.
When the presenting part is at a very low station, a transperineal ultrasound may be helpful to determine the position of the occiput. The ISUOG recommends that position be defined using a clock face, with positions from 330 h to 830 h being indicative of OP and positions from 930 h to 230 h being indicative of OA.11 The small remaining slivers on the clock face indicate an occiput transverse position (FIGURE).11
Continue to: Approaches to managing the OP position