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Fetal Positions

Fetal position plays a very important role in determining the course of labor, and in whether the baby will ultimately fit through the mother's pelvic opening. When we consider the best position of the baby for delivery, we begin to understand why some positions make vaginal delivery more difficult, some make it more dangerous, and why some positions are completely undeliverable.

During normal vaginal delivery, there is a relatively tight fit between the largest part of the baby, almost always its head, and the mother's pelvic opening. If the baby can position his or her head in such a way as to present the smallest possible diameter, labor will progress more smoothly.

Neither the fetal nor the infant head is perfectly round. When you dress a baby, you take this into account without even thinking about it. It is much easier to pull on those cute little T-shirts starting at the back of the baby's head, because the head is naturally egg shaped. Starting over the "point" at the back of the head makes it easier to slip clothes on.

In exactly the same way, starting labor with the "point" at the back of the baby's head leading the way makes it easier for the head to slip through the pelvic opening. In the ideal position for labor, the baby is head down, facing the mother's back, with its chin tucked on its chest. This position is known as occiput anterior, which signifies that the occiput (back) of the fetal skull rests against the anterior (front) portion of the mother's pelvis.

Of course, lots of babies don't cooperate. Their chins may not be tucked, a position known as deflexed head, or their heads may be turned to one side or another in the occiput transverse position. Some may face away from their mothers' backs in the occiput posterior position. In all these cases, labor is likely to be slower and more difficult, because the position of the baby's head creates a slightly larger diameter that must pass through the pelvic opening. When the fetus is in the occiput posterior position, the mother is likely to experience back labor, feeling the contractions more in her back than in the front of her body.

Then, there are some babies who are totally uncooperative. They choose to assume the breech position (this position is also referred to as a breech presentation). This means that the baby's bottom, not the head, comes first, which makes for a more difficult delivery than those in which the baby assumes one of the other possible positions.

It's easy to understand why the breech presentation is potentially dangerous. On the one hand, the breech (the baby's rear end) may easily pass through the pelvic opening because of its smaller diameter. However, the head, which is always larger, may then get trapped in the pelvic opening, which constitutes an obstetric emergency.

A baby cannot breathe until the head is successfully delivered. Therefore, strenuous efforts must be made to deliver the head as quickly as possible in a breech presentation. Unfortunately, these same efforts, which are required to save the baby's life, may cause serious, permanent injury. This is the main reason that breech vaginal deliveries are considered hazardous. For more detail on breech presentations, see Chapter 26.

It's interesting to note that a baby that can only be delivered with great difficulty from the breech position would fit easily through the pelvic opening, if it arrived head first. This is true because the head would have had an opportunity to mold itself to the pelvic opening during the long hours of labor. This same head has had no time to mold if the breech has been delivered first. Furthermore, the umbilical cord is compressed between the baby's body and the walls of the vagina, depriving the baby of oxygen.

Finally, there are some positions that are just undeliverable, no matter how long or strong the contractions are. A transverse lie is such a position. The fetal back may be pressing against the cervix, or perhaps a shoulder is coming through first. Clearly, the entire baby will never fit through the pelvic opening back first. If the baby cannot be coaxed into assuming a more favorable position, a C-section is the only choice.

There is no reason to worry if your baby's head is not down before 36 weeks of gestation. Before then, there is a lot of room for the baby to turn around, and it will most likely do just that. However, if your practitioner finds the baby to be in the breech or transverse position in the last few weeks of pregnancy, he or she can try to manually turn the baby to the head-down position. This procedure is called version and it is successful more than 50 percent of the time. Version will be considered in detail in Chapter 26.

Remember, fetal position is just one of many factors that determine the course and outcome of labor. Because parents and practitioners have little or no control over these factors, everyone involved must be flexible. In labor, the baby runs the show. Parents and caregivers can only respond to its demands.