Of all the possible presentations, the most serious problems for both mother and baby during labor and delivery occur in a transverse presentation, where the baby is lying longways in the uterus across the opening to the birth canal. Most transverse fetuses cannot be delivered vaginally because they are too wide to pass through the birth canal. And as the uterus continues to contract while trying to push the fetus into the birth canal, it can rupture-a possibly life-threatening situation for both mother and baby.
Before labor, transverse presentations are usually not dangerous because the baby is probably in the process of changing from a breech to a cephalic presentation or vice versa. But during labor, a transverse presentation should be converted to either a cephalic (head-first) or breech (buttocks-first) presentation, or a cesarean section should be performed. The process of manipulating the fetus into a cephalic presentation is called an external cephalic version and is discussed in.
A compound presentation occurs when the baby's arm or leg is next to the main presenting part, usually the head. Labor can usually proceed normally without any manipulation, which might harm the baby or cause the umbilical cord to slip through the cervix. Usually as labor progresses, the compound presenting part will retract and the baby's head will ultimately present. Sometimes the obstetrician will pinch the baby's finger to stimulate a reflex contraction which will relieve the compound presentation.
The baby's head can be in several positions that can affect labor. To determine the position of the baby, the doctor feels for the head in relation to the mother's pelvis. The key to getting the head through the pelvis is to pass the smallest part of the head through the smallest parts of the pelvis. For most women, this means the baby delivers in what's known as the occiput anterior position, or face-down, with the back of the head against the front of the mother's pelvis.
Other positions may keep the head from passing through, depending on:
- the shape of the mother's pelvis;
- the shape of the baby's head;
- how much the baby's head can mold or change shape; and
- how much the mother's pelvic floor muscles can contract and relax.
The occiput posterior position, where the baby is facing up, with the back of its head against the back of the mother's pelvis, is the most common head position to cause problems during delivery. Several factors increase the risk of an occiput posterior position, including an android or anthropoid pelvis in the mother. In most cases, no intervention is necessary to deliver a baby whose head is in the occiput posterior position. But if labor does not progress normally, in spite of adequate contractions and pushing by the mother, the baby's head can sometimes be rotated into the anterior or face-down position, either manually or with forceps. If this can't be done and the baby is still not progressing through the birth canal, preparations for cesarean section should be made.
Brow or face presentations, where the fetus enters the birth canal face-first or brow-first, are much less common than occiput anterior and occiput posterior presentations, occurring once in every 500 deliveries. They are more common when:
- the fetal membranes rupture prematurely;
- the baby's head is large; and
- the mother has previously given birth.
Most brow presentations change to occiput anterior/posterior or face presentations on their own, and many face presentations correct themselves before the second stage of labor (the pushing phase). If labor continues to progress during the second stage, vaginal delivery may be attempted. However, if labor is arrested there should be no attempt to manipulate the head either manually or with forceps. Instead, the baby should be delivered by cesarean section.