What is a contraction stress test?
The contraction stress test — also called a stress test or an oxytocin challenge test — may be done during pregnancy to measure the baby's heart rate during uterine contractions. Its purpose is to make sure the baby can get the oxygen he needs from the placenta during labor.
(For a variety of reasons, the contraction stress test is rarely done these days. In most cases, practitioners are able to evaluate the baby more quickly and safely using the biophysical profile or nonstress test or both.)
During contractions, the flow of blood and oxygen to the placenta temporarily slows down. If your placenta is healthy, it has extra stores of blood ready to provide the baby with the oxygen he needs during contractions. So if everything is okay, your baby's heart won't slow down during or after a contraction. But if the placenta isn't functioning properly, your baby won't get enough oxygen and his heart will beat more slowly after a contraction.
The contraction stress test is more cumbersome, expensive, and risky than other similar tests, so it's not done very often anymore. But if you're having a high-risk pregnancy, your healthcare practitioner might recommend it as you get close to your due date.
What's the procedure like?
You'll be asked not to eat or drink anything for six to eight hours before the test, on the slim chance that the results will call for an emergency c-section. (Emptying your bladder shortly before the test is still a good idea.)
When it's time for the test, you lie on your left side. A technician straps two devices to your belly: One monitors your baby's heartbeat; the other records contractions in your uterus. A machine records your contractions and your baby's heartbeat as two separate lines on graph paper.
The test lasts until you've had three contractions in a ten-minute period, each lasting 40 to 60 seconds. This can take up to two hours. You may barely feel the contractions or they may feel a bit like menstrual cramps; they shouldn't be strong enough to induce labor.
If you don't have contractions on your own during the first 15 minutes, your practitioner may try to get them going by giving you a small dose of synthetic oxytocin (Pitocin) in an IV or by asking you to stimulate your nipples, which releases natural oxytocin.
When the test is over, you'll need to stick around until your contractions stop or go back to their pretest level.
What do the results mean?
If your baby's heartbeat doesn't slow down in response to your contractions, he's probably doing fine. This is called a normal or negative result. In this case, you'll either wait to go into labor naturally or have the test again in a week.
If your baby's heart beats more slowly after more than half of your contractions, the test result is positive, signaling that your baby may be under stress. In that case, your practitioner might recommend delivery right away, either cesarean section if your baby couldn't tolerate any contractions or appears to be in imminent danger, or by inducing labor if your cervix is soft and ready or starting to dilate.
The stress test is very reliable when it indicates that everything is okay, but not so reliable when it indicates that there might be a problem. Your chances of getting a false positive result (indicating a problem when there isn't one) could be as high as 30 percent.
Are there any risks from this procedure?
Depending on the skill of the person administering the test, there's a small risk that your uterus will be hyperstimulated, causing contractions so strong and frequent that they cut off blood flow to the baby. (This may be more of a risk from nipple stimulation, since it's harder to control than Pitocin, but can be caused by either.)
The test may also stimulate premature labor. This can happen if you get too much Pitocin too quickly (sensitivity to the drug can vary from woman to woman) or if you stimulate your nipples for too long. That's why the test is not recommended if you have any risk factors preterm labor, if you have a placenta previa, or if you've had a "classical" c-section, in which the incision goes up and down rather than across the uterus.
For all these reasons, most practitioners will avoid the contraction stress test if possible and will recommend a less invasive or complicated procedure, such as the biophysical profile or nonstress test, instead.