You’ve been in labor for several hours and you are anxious to find out about your progress. You might get very excited when your practitioner agrees that it's time to check your cervix. You watch him or her don the sterile glove, you feel fingers being inserted into your vagina, and you hear the approving announcement that you have dilated to 4 cm. But what on earth did the practitioner just do? What does the number mean?
When doctors or midwives do vaginal exams, they are able to feel three different things with their fingers. They measure cervical dilation, which is the amount that the cervix has opened, and fetal station, which is the relationship between the largest part of the fetal head and the midpoint of the maternal pelvis. If the cervix is dilated far enough, they can even feel the position of the baby’s head. (See Chapter 12 for details on fetal positions.)
Cervical dilation is easy to measure. After identifying the cervical opening overlaying the baby’s head, the practitioner slips one or two fingers into the opening cervix. If only one finger can be admitted, the cervix is approximately 1 cm. dilated. If two fingers can be placed in the opening, the cervical dilatation is 2 cm. or greater, depending on how wide the fingers can spread. As the cervix approaches full dilatation, an ever-smaller rim of the cervix can be felt at the outer edge of the fetal head. At 10 cm., no cervix can be felt at all.
As the cervix dilates, it is not only opening wider; it is also thinning out, or effacing. Before labor begins, the cervix is 2 to 3 cm. long. As labor progresses, it becomes shorter. This effacement is described in relation to the original length of the cervix. When it is half as long as the original measurement, the cervix is 50 percent effaced. When it is paper thin, it is 100 percent effaced.
Fetal station is measured by determining the location of the largest part of the fetal head in relation to the midpoint of the mother's pelvis. If the largest part is at the midpoint of the maternal pelvis, the station is 0. If the biparietal diameter of the fetal head is 1 cm. above the midpoint, the station is -1. As labor progresses, the baby’s head descends into the pelvis, ultimately culminating at +5; at that point, the fetal head is at the vaginal opening. When your practitioner explains your progress, he or she will refer to both these numbers. For example, if you are examined after several hours of latent phase labor, your exam may show 4 cm., 0 station. Several hours later you may have progressed to 7 cm., +1 station. This means that in the intervening time, the cervix has dilated an additional 3 cm. and the baby’s head has descended 1 cm.
If the cervix is sufficiently dilated, your practitioner may be able to determine the position of the baby’s head. This is done by feeling for landmarks on the head. The bones that make up the baby’s skull are separate and not fused together, as they are in the adult skull. The space between each bone is called a suture, which can be clearly felt between the bones. At the places where several bones adjoin, there are even larger spaces called fontanelles. The fetal head has two fontanelles, anterior and posterior, commonly referred to as "soft spots." By feeling the location of the fontanelles and the sutures, your practitioner may be able to determine which direction your baby is facing (occiput anterior or posterior, for example) and whether its chin is tucked on its chest.
In most cases, it is not possible to feel the landmarks on the baby’s head clearly and accurately until the membranes of the amniotic sac rupture. When the sac is intact, the membranes overlaying the baby’s head feel like a balloon, and the details of the head cannot be discerned.
In early labor, especially if the baby’s head is high, the cervical exam may be a bit uncomfortable. As labor progresses, the cervix dilates more and the head descends closer to the vagina. Consequently, each additional exam is usually easier for the mother than the last.
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