How the Uterus Contracts

You recognize contractions when you feel them because the pain and pressure are unmistakable. But what are contractions? How do they cause the cervix to dilate? What happens to the baby during a contraction?

In order to understand contractions, it is important to consider how the uterus acts on the baby. Think of the uterus as a muscular bag that is made up of involuntary muscle. You cannot flex or release this type of muscle as you can the muscles that control the movements of your limbs. Therefore, you can't strengthen these involuntary muscles by exercising before labor. If the hormone oxytocin is present in sufficient quantities, it will cause the involuntary muscle of the uterus to contract in a regular pattern.

The involuntary muscle of the uterus is divided into two parts. The upper part is the active segment; it contracts and pushes against the baby. The lower part of this muscular bag is known as the passive segment. This part actually remains relaxed during the contraction. The net effect is to push the fetal head against the cervix. Repeated pressure of the fetal head on the cervix causes the cervix to thin out (efface) and then retract around the fetal head. The baby is not so much forced out of the bag, as the bag pulls up around its head.

The average diameter of a fetal head is 10 cm. That's why the cervix is not considered to be fully dilated until 10 cm. is reached. At that point, the cervix has thinned out so much that there is no cervix left to hold back the fetal head, and the baby can be pushed out. If you start pushing before 10 cm. dilation is reached, you may damage the cervix.

When the involuntary muscle of the active segment contracts, it also compresses the blood vessels that travel through the uterine wall and carry oxygen to the placenta. This means that during the contraction, no oxygen can be transferred to the baby. It's as if the baby holds its breath for the length of the contraction. However, the placenta is designed to compensate for this under normal circumstances. Enough oxygen is transported to the baby between contractions to allow the baby to tolerate each contraction without any difficulty.

If there is a problem with the placenta-perhaps the baby is overdue and the placenta is a bit "worn out"-the baby may not tolerate the contractions nearly so well. In such a case, the baby may receive the minimally acceptable amount of oxygen between contractions, but may have no reserves with which to "hold its breath" during contractions. In this case, the baby's heart rate may drop immediately after each contraction. Continuous fetal heart rate monitoring can alert your practitioner to this potentially dangerous situation. Often, administering supplemental oxygen to the mother will correct the problem.

If this problem persists despite additional oxygen, the doctor or midwife must consider the possibility that the baby will remain oxygen-deprived for the rest of labor. This condition is known as fetal distress. If there are many hours of labor remaining (perhaps the mother's cervix is dilated to only 5 cm.), a C-section would most likely be indicated to prevent the possibility of brain damage caused by chronic oxygen deprivation.

back to Pregnancy FAQs