The colloquial term “sunny side up baby” refers to the Occiput Posterior (OP) fetal presentation, where the baby’s head is positioned face-up, facing the mother’s abdomen. While this presentation is common early in labor, a persistent OP position significantly complicates the birthing process, often leading to a prolonged delivery. The difficulties stem from a mechanical mismatch between the fetal head and the maternal pelvis, impairing the natural rotation and descent required for a successful vaginal birth.
Defining the Occiput Posterior Position
The fetal position is described relative to the mother’s pelvis, based on the location of the occiput (the back of the baby’s skull). The optimal position for delivery is the Occiput Anterior (OA) position, where the occiput is oriented toward the front of the pelvis. This ideal face-down alignment means the baby is facing the mother’s spine, allowing the fetal head to naturally flex and present its smallest diameter to the birth canal.
In contrast, the Occiput Posterior (OP) position means the occiput is toward the mother’s back, and the baby is facing the mother’s abdomen. This malposition occurs in an estimated 10 to 35% of women at the onset of labor. However, most of these fetuses spontaneously rotate to the anterior position as labor progresses. Only about 5% of babies remain in the OP position at the time of delivery, but this persistent malposition causes the majority of complications.
The Mechanical Obstruction of Labor
The primary reason the OP position complicates labor is a failure of the fetal head to present its smallest, most compressed diameter to the pelvis. In the optimal Occiput Anterior position, the fetal head flexes deeply, allowing the narrowest part, the suboccipitobregmatic diameter (approximately 9.5 cm), to navigate the birth canal. This tight flexion is a requirement for the cardinal movements of labor, including internal rotation under the pubic bone.
When the baby is in the OP position, the head often becomes deflexed, or partially extended, which prevents proper engagement and descent. This deflexion causes the wider occipitofrontal diameter (approximately 11.5 cm) to become the presenting part. This larger diameter creates significant mechanical resistance against the narrow bony pelvis, making it much harder for the baby to pass through.
Furthermore, the OP position interferes with the internal rotation that normally guides the occiput under the mother’s pubic bone. Instead of rotating 45 degrees to the front, the occiput must rotate a longer, more difficult 135 degrees, or fail to rotate entirely. This mechanical difficulty often results in labor dystocia (abnormally slow or difficult labor progression). The increased friction and pressure during this obstructed passage contribute directly to prolonged labor and maternal exhaustion.
Increased Maternal Risks and Interventions
The mechanical obstruction created by a persistent OP position leads directly to a higher rate of adverse maternal outcomes and medical interventions. Women with this fetal presentation often experience a significantly protracted labor, with both the first and second stages lasting longer than in an Occiput Anterior delivery. This prolonged course is a common indication for medical intervention, as the uterus can become fatigued, which may increase the risk of postpartum hemorrhage.
A common symptom associated with the OP position is severe “back labor,” which is intense pain concentrated in the mother’s lower back. This pain is caused by the baby’s hard occiput pressing directly against the maternal sacrum and coccyx bones during contractions and descent.
Due to the difficulty in achieving a spontaneous vaginal delivery, the rate of operative intervention is notably higher with a persistent OP presentation. These interventions include the use of instruments like vacuum extractors or forceps, required in approximately one-fourth of these deliveries. Most significantly, the rate of Cesarean delivery is dramatically increased, often due to failure of labor to progress, with the risk being over 13 times higher compared to an OA position. Delivery complications can also increase the risk of third or fourth-degree perineal lacerations and anal sphincter injury.
Clinical Strategies for Resolution
Healthcare providers employ various strategies to manage and attempt to correct an OP presentation during labor. Early in labor, non-invasive techniques focus on using gravity and maternal positioning to encourage the baby’s head to rotate. The mother may be encouraged to try positions such as hands-and-knees, lateral lying, or leaning forward over a birthing ball. These positions are intended to use the open space in the pelvis to facilitate the baby’s rotation into the more favorable anterior position.
If the baby remains in the OP position into the second stage of labor, a more direct intervention may be considered to facilitate delivery. The obstetric provider may attempt a manual rotation of the fetal head, which involves placing a hand into the vagina to physically turn the baby’s occiput toward the anterior pelvic quadrant. This procedure aims to mimic the natural rotation process and improve the chance of a successful vaginal delivery. If manual rotation is unsuccessful or if labor progress is arrested, the provider must then decide between an operative vaginal delivery using a vacuum or forceps, or proceeding directly to a Cesarean section.