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 deliver vaginally safely.
Most babies are going to present, at time of delivery, head-first. When a baby is head-first, they can be making that slow descent into the pelvic inlet before labor even starts.
When you think about the pelvis and the baby coming through the pelvis, they’re always going to try to put the smallest part in the pelvis first. And for that first initial part of descent—coming in sideways—the baby’s going to fit the easiest. As the baby is experiencing forces of labor, the baby’s head does then have to flex for it to fit.
Most babies, when they’re making this transition, are going to be in an anterior rotation where the face is the closest to mom’s back. A malposition—the posterior position—is when the baby’s face is towards mom’s belly.
For the baby then to continue its passage through the birth canal, it’s going to be rotating and coming through. So when the baby’s born, typically they’re looking a little bit to the right, a little bit to the left, but they’re looking down. Once the head is delivered, then the shoulders need to be delivered. Some of you may have heard the term shoulder dystocia; that’s when one of the shoulders gets caught behind the pelvis. It can be an emergency situation, and it needs to be remedied very quickly.
After the shoulders are delivered, the rest of the body comes out very easily.