Fetal descent is impacted by the maternal pelvis. The maternal pelvis is a bony structure, and it forces the baby to make several movements in order for it to fit through and to delivery vaginally safely.
The most common malpresentation that we see is posterior position. Doctors sometimes refer to it as “sunny side up.” So, that’s when the baby’s head is actually facing towards mom’s belly, and when the baby delivers vaginally in that position, the baby is actually looking at the ceiling—looking up, or sunny side up. Babies can be delivered vaginally. But, sometimes when they’re in that posterior position, they can have labor dystocia, and they can get stuck. Things that impact that can be the baby’s size or the mom’s size. While some healthy pregnancies can tolerate prolonged labor—additional hours contracting—some babies cannot. So, when that happens, you can see the baby’s head gets caught up on the tailbone. If the baby’s head is too large, it’s not going to come through the birth canal. When a baby’s suffering from a prolonged labor, and they’re having contraction after contraction for hours, they can suffer from hypoxia [and trauma].