During pregnancy, the fetus can be positioned in many different ways inside the mother's uterus. The fetus may be head up or down or facing the mother's back or front. At first, the fetus can move around easily or shift position as the mother moves. Toward the end of the pregnancy the fetus is larger, has less room to move, and stays in one position. How the fetus is positioned has an important effect on delivery and, for certain positions, a cesarean delivery is necessary. There are medical terms that describe precisely how the fetus is positioned, and identifying the fetal position helps doctors to anticipate potential difficulties during labor and delivery.
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 (breech presentation), shoulder, or face leads the way.
Position refers to whether the fetus is facing backward (occiput anterior) or forward (occiput posterior). The occiput is a bone at the back of the baby's head. Therefore, facing backward is called occiput anterior (facing the mother’s back and facing down when the mother lies on her back). Facing forward is called occiput posterior (facing toward the mother's pubic bone and facing up when the mother lies on her back).
Lie refers to the angle of the fetus in relation to the mother and the uterus. Up-and-down (with the baby's spine parallel to mother's spine, called longitudinal) is normal, but sometimes the lie is sideways (transverse) or at an angle (oblique).
For these aspects of fetal positioning, the combination that is the most common, safest, and easiest for the mother to deliver is the following:
Head first (called vertex or cephalic presentation)
Facing backward (occiput anterior position)
Spine parallel to mother's spine (longitudinal lie)
Neck bent forward with chin tucked
Arms folded across the chest
If the fetus is in a different position, lie, or presentation, labor may be more difficult, and a normal vaginal delivery may not be possible.
Variations in fetal presentation, position, or lie may occur when
The fetus is too large for the mother's pelvis (fetopelvic disproportion).
The uterus is abnormally shaped or contains growths such as fibroids.
The fetus has a birth defect.
There is more than one fetus (multiple gestation).
Position and Presentation of the Fetus
Toward the end of pregnancy, the fetus moves into position for delivery. Normally, the presentation is vertex (head first), and the position is occiput anterior (facing toward the pregnant person's spine) and with the face and body angled to one side and the neck flexed. Variations in fetal presentations include face, brow, breech, and shoulder. Occiput posterior position (facing forward, toward the mother's pubic bone) is less common than occiput anterior position (facing backward, toward the mother's spine). |
Variations in Fetal Position and Presentation
Some variations in position and presentation that make delivery difficult occur frequently.
Occiput posterior position
In occiput posterior position (sometimes called sunny-side up), the fetus is head first (vertex presentation) but is facing forward (toward the mother's pubic bone—that is, facing up when the mother lies on her back). This is a very common position that is not abnormal, but it makes delivery more difficult than when the fetus is in the occiput anterior position (facing toward the mother's spine—that is facing down when the mother lies on her back).
When a fetus faces up, the neck is often straightened rather than bent,which requires more room for the head to pass through the birth canal. Delivery assisted by a vacuum device or forceps or cesarean delivery may be necessary.
Breech presentation
In breech presentation, the baby's buttocks or sometimes the feet are positioned to deliver first (before the head).
When delivered vaginally, babies that present buttocks first are more at risk of injury or even death than those that present head first.
The reason for the risks to babies in breech presentation is that the baby's hips and buttocks are not as wide as the head. Therefore, when the hips and buttocks pass through the cervix first, the passageway may not be wide enough for the head to pass through. In addition, when the head follows the buttocks, the neck may be bent slightly backwards. The neck being bent backward increases the width required for delivery as compared to when the head is angled forward with the chin tucked, which is the position that is easiest for delivery. Thus, the baby’s body may be delivered and then the head may get caught and not be able to pass through the birth canal. When the baby’s head is caught, this 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 breech babies than among those presenting head first.
In a first delivery, these problems may occur more frequently because a woman’s tissues have not been stretched by previous deliveries. Because of risk of injury or even death to the baby, cesarean delivery is preferred when the fetus is in breech presentation, unless the doctor is very experienced with and skilled at delivering breech babies or there is not an adequate facility or equipment to safely perform a cesarean delivery.
Breech presentation is more likely to occur in the following circumstances:
Labor starts too soon (preterm labor).
There is more than one fetus (multiple gestation).
The uterus is abnormally shaped or contains abnormal growths such as fibroids.
The fetus has a birth defect.
Other presentations
In face presentation, the baby's neck arches back so that the face presents first rather than the top of the head.
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. These presentations often change to a vertex (top of the head) presentation before or during labor. If they do not, a cesarean delivery is usually recommended.
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.
