Position refers to whether the fetus is facing rearward (toward the woman’s back—that is, face down when the woman lies on her back) or forward (face up).
Presentation refers to the part of the fetus’s body that leads the way out through the birth canal (called the presenting part). Usually, the head leads the way, but sometimes the buttocks or a shoulder leads the way.
The most common and safest combination consists of the following:
If the fetus is in a different position or presentation, labor may be more difficult, and delivery through the vagina may not be possible.
Position and Presentation of the Fetus
There are several abnormal presentations.
In occiput posterior presentation (also called sunny-side up), the fetus is head first but is facing up (toward the mother's abdomen). It is the most common abnormal position or presentation.
The buttocks or sometimes the feet present first. Breech presentation occurs in 3 to 4% of full-term deliveries. It is the second most common type of abnormal presentation.
When delivered vaginally, babies that present buttocks first are more likely to be injured than those that present head first. Such injuries may occur before, during, or after birth. The baby may even die. Complications are less likely when breech presentation is detected before labor or delivery.
Breech presentation is more likely to occur in the following circumstances:
Sometimes the doctor can turn the fetus to present head first by pressing on the woman’s abdomen before labor begins, usually after 37 weeks of pregnancy. Some women are given a drug (such as terbutaline) to prevent labor from starting too soon. If labor begins and the fetus is in breech presentation, problems may occur.
The passageway made by the buttocks in the birth canal may not be large enough for the head (which is wider) to pass through. In addition, when the head follows the buttocks, it cannot be molded to fit through the birth canal, as it normally is. Thus, the baby’s body may be delivered and the head may be caught inside the woman. When the baby’s head is caught, it puts pressure on the umbilical cord in the birth canal, so that very little oxygen can reach the baby. Brain damage due to lack of oxygen is more common among babies presenting buttocks first than among those presenting head first.
In a first delivery, these problems may occur more frequently because the woman’s tissues have not been stretched by previous deliveries. Because the baby could be injured or die, cesarean delivery is preferred when the fetus is in breech presentation unless the doctor is very experienced with and skilled at delivering breech babies.
In face presentation, the neck arches back so that the face presents first.
In brow presentation, the neck is moderately arched so that the brow presents first.
Usually, fetuses do not stay in a face or brow presentation. They often correct themselves. If they do not, forceps, vacuum extractor, or cesarean delivery may be used.
In transverse lie, the fetus lies horizontally across the birth canal and presents shoulder first. A cesarean delivery is done, unless the fetus is the second in a set of twins. In such a case, the fetus may be turned to be delivered through the vagina.
The fetus is positioned normally (head first) for delivery, but the fetus’s shoulder becomes lodged against the woman’s pubic bone as the fetus’s head comes out. Consequently, the head is pulled back tightly against the vaginal opening. The baby cannot breathe because the chest and umbilical cord are compressed by the birth canal. As a result, oxygen levels in the baby’s blood decrease.
Shoulder dystocia is not common, but it is more common when any of the following is present:
When this complication occurs, the doctor quickly tries various techniques to free the shoulder so that the baby can be delivered vaginally. Sometimes when these techniques are tried, the nerves to the baby’s arm are damaged or the baby’s arm bone or collarbone may be broken. An episiotomy (an incision that widens the opening of the vagina) may be done to help with delivery.
If these techniques are unsuccessful, the baby may be pushed back into the vagina and delivered by cesarean. If all of these techniques are unsuccessful, the baby may die.