The transverse position in pregnancy is a rare fetal orientation where the baby lies horizontally across the uterus instead of aligning vertically (the typical head-down or feet-down position). This sideways orientation means the baby’s spine is perpendicular to the mother’s spine. While common earlier in gestation due to ample space, this position becomes a concern in the third trimester as the due date approaches. A persistent transverse position close to term (after 37 weeks) poses complications that make a vaginal birth unsafe.
Anatomy of Transverse Position
Fetal lie describes the relationship between the long axis of the fetus and the long axis of the mother. In a transverse lie, these axes cross at roughly a 90-degree angle, contrasting with the longitudinal lie where they are parallel. The presenting part, closest to the birth canal, is typically the baby’s shoulder (shoulder presentation).
The position is further described by the location of the baby’s back, or dorsum, which can be superior (back up) or inferior (back down). This orientation is distinct from an oblique lie, where the fetal axis is positioned at an angle that is not quite vertical or horizontal. A persistent transverse lie prevents the baby from entering the pelvic inlet for delivery.
Understanding the Contributing Factors
Several conditions can increase the likelihood of a fetus maintaining a transverse position late in pregnancy. When the uterus has been significantly stretched from multiple previous pregnancies (grand multiparity), the uterine muscles may become lax. This laxity provides the fetus with excessive room to move and prevents it from settling into a longitudinal position.
An abnormal volume of amniotic fluid can also be a factor. Too much fluid (polyhydramnios) gives the baby too much freedom to shift positions, while too little fluid (oligohydramnios) can prevent the baby from turning out of a transverse position once settled. Structural issues within the uterus, such as uterine fibroids or a bicornuate uterus, can physically interfere with the space needed for the baby to orient vertically.
The location of the placenta can also play a role, especially if placenta previa is present, where the placenta partially or completely covers the internal opening of the cervix. This blocks the natural pathway into the pelvis, forcing the fetus to remain in a transverse orientation. In a multiple gestation pregnancy, such as twins, the reduced space often causes one or both fetuses to adopt a non-longitudinal lie.
Monitoring, Management, and Intervention
The diagnosis of a transverse position is often suspected during a physical examination in the third trimester when the fundal height is lower than expected. An ultrasound scan is then used to confirm the orientation of the fetus, typically after 32 to 36 weeks of gestation. Close monitoring is necessary because if the membranes rupture while the baby is sideways, there is a high risk of the umbilical cord prolapsing, which is an obstetrical emergency.
If the transverse position persists, an attempt may be made to manually turn the baby through the abdominal wall in a procedure called an External Cephalic Version (ECV). This procedure is typically considered around 37 weeks of pregnancy to encourage the fetus to move into a head-down position before labor begins. ECV is often performed in a hospital setting using medication to relax the uterine muscle, increasing the chances of a successful turn.
If ECV is unsuccessful or medically contraindicated, a planned Cesarean section is the safest course of action. A persistent transverse lie at the onset of labor makes vaginal delivery unsafe because the baby cannot navigate the pelvis in this orientation. Cesarean section is mandatory in these cases to avoid complications like uterine rupture and shoulder dystocia. The Cesarean delivery is usually scheduled after 39 weeks of gestation if the position has not changed.