Fetal presentation refers to the position of the baby inside the uterus, specifically which part is nearest to the mother’s cervix for delivery. The most common orientation for a vaginal birth is the cephalic presentation, where the baby’s head is pointing down toward the birth canal. This head-down position is optimal because the head is the largest part of the baby’s body. Once the head is delivered, the rest of the body usually follows. A non-cephalic presentation late in pregnancy can complicate the birthing process, often necessitating alternative delivery plans.
The Standard Timeline for Fetal Positioning
The majority of babies naturally assume the head-down position during the third trimester of pregnancy, most commonly between 32 and 36 weeks of gestation. Before this period, a baby may move in and out of the cephalic position, but the space inside the uterus begins to limit movement as the baby grows. By the 36th week, the likelihood of a baby spontaneously turning from a non-cephalic to a cephalic position significantly decreases.
The process of the baby settling into the pelvis is known as engagement, which is distinct from simply turning head-down. Engagement is defined as the widest part of the baby’s head successfully passing into the mother’s pelvic inlet. For women pregnant for the first time, engagement often happens a few weeks before the due date. This descent can cause a noticeable shift in the mother’s abdomen, sometimes referred to as “lightening”.
In contrast, women who have given birth previously may not experience engagement until they are actively in labor. This is because the pelvis and uterine muscles have already been stretched and may be less restrictive. The timing of both the turn and the subsequent engagement can vary widely, but a health care provider will routinely check the fetal position starting around 36 weeks.
Understanding Non-Cephalic Presentations
When a baby remains in a position other than head-down near term, it is called a non-cephalic presentation, or malpresentation. The most common type is a breech presentation, which occurs in about three to four percent of pregnancies at full term. A breech baby has its feet or buttocks positioned to exit the birth canal first.
Types of Breech Presentation
There are three primary types of breech presentation, differentiated by the baby’s leg position:
- Frank breech: The baby’s buttocks are presenting, with both legs extended straight up toward the head.
- Complete breech: The baby is sitting cross-legged, with both hips and knees flexed.
- Footling breech: One or both feet are positioned below the baby’s buttocks and would present first.
A transverse lie is another non-cephalic position, where the baby lies horizontally across the uterus, with the shoulder often presenting first. Both breech and transverse presentations carry higher risks during a vaginal delivery, such as the possibility of the umbilical cord prolapsing or the baby’s head getting stuck. Health care providers diagnose these positions through abdominal palpation and confirm them with an ultrasound, often leading to a discussion about a planned cesarean delivery.
Clinical Interventions for Repositioning
When a baby is confirmed to be in a breech or transverse position after 36 weeks, a medical procedure called External Cephalic Version (ECV) may be offered. ECV is an attempt by a health care provider to manually rotate the baby from the outside of the mother’s abdomen into the cephalic position. The procedure is usually performed after 37 weeks of gestation, as the baby is less likely to spontaneously revert back to the non-cephalic position.
The ECV procedure involves the application of firm, guided pressure to the mother’s abdomen to encourage the baby to perform a somersault. To help relax the uterine muscle and increase the chance of success, the mother is often given a drug called a tocolytic. The overall success rate for ECV is approximately 58% to 65%, and it significantly lowers the probability of needing a cesarean delivery.
ECV is a hospital-based procedure and is always performed where an emergency cesarean section can be carried out immediately, although serious complications are rare. It is generally avoided if there are concerns like low amniotic fluid, a known placental issue such as placenta previa, or if the mother is carrying multiples. If the ECV is successful, the mother can then proceed with plans for a vaginal birth.