What is a contraction stress test?
In this procedure, your baby’s heart rate is measured in response to the uterus when it contracts. These contractions are mild and induced. Every contraction you have squeezes the baby and gives the doctors a chance to see how he or she will stand up to the physical challenges involved in labor. As stressful as that may sound, for most babies the test presents no problem.
A contraction stress test (CST) can reveal whether your baby has an abnormal heart rate during contractions — a distinct pattern of slowing heartbeats during and immediately following a contraction — that may indicate distress. In this way, the test may help predict how well your baby will do during labor and delivery.
When is the test done?
This test may be recommended if other fetal evaluation tests — a nonstress test or a biophysical profile — reveal no change in the fetal heart rate when your baby moves. Such results may be a sign that something is amiss.
A CST may also be recommended if you are at risk for stillbirth or if yours is a high-risk pregnancy. You are considered to be at high risk if you have diabetes or high blood pressure or if you’ve had complications with a previous pregnancy.
If you have multiple high-risk factors or particularly worrisome conditions, your health care provider may advise that testing begin as early as your 26th to 28th week of pregnancy. Otherwise testing usually takes place around the 32nd to 34th week of pregnancy. If you’re in your 40th week or later, your practitioner will probably order a round of tests that includes a nonstress test and a biophysical profile.
What’s the procedure like?
You’ll lie on an examination table and contractions will be brought on in one of two ways. You may be asked to gently massage your nipples through your clothing. This stimulation releases oxytocin, a hormone that is produced naturally during labor. It may be all that’s needed to kick-start contractions if the CST is done late in your pregnancy.
But if nipple stimulation doesn’t do the job, you may get a low intravenous dose of a synthetic form of oxytocin called pitocin to bring on contractions. This is sometimes called an Oxytocin Challenge Test or OCT.
Before you experience contractions, you’ll have two separate fetal monitoring devices placed on your abdomen with elastic straps. These electronic monitors measure both your contractions and your baby’s heart rate and reveal the findings through a printout. The contraction stress test may take as long as one to two hours. Most women say the test is uncomfortable but not painful; some do not even feel the contractions.
What do the results mean?
A normal or negative response to this test — meaning your baby’s heartbeat doesn’t slow in response to contractions — indicates that your baby is probably getting enough oxygen and should handle the challenges of labor just fine. Still, your practitioner may want to repeat the test every week just to play it safe. An abnormal or positive response is when your baby’s heart rate does drop during or immediately after a contraction. It suggests more testing may be necessary.
Don’t be alarmed. This test is very good at indicating when all is well, but not as accurate at predicting if things are wrong. In other words, it has a high false-positive rate. That’s why follow-up tests are typically performed following this procedure.
Depending on your specific situation, your health care provider may forgo further testing and instead recommend immediate delivery, either by inducing labor or by cesarean section. She may recommend this if she suspects, based on all the information available, that your baby is in fetal distress.
Are there any risks with this procedure?
There’s a small risk that a CST can trigger what is known as hyperstimulation, when the contractions are so strong and frequent they cut off full blood flow to your baby. The test may also stimulate premature labor. In both these cases, immediate delivery or medical intervention may be necessary.
Because of such dangers, the contraction stress test is not recommended if you’re at increased risk of preterm labor or premature membrane rupture, if you have a history of extensive uterine surgery, or if you have placenta previa, meaning that the placenta is blocking the birth canal. It’s also not recommended for women who’ve had a cesarean section.
March of Dimes. Placental Conditions. June 2010. http://www.marchofdimes.com/professionals/14332_1154.asp
American Congress of Obstetricians and Gynecologists. Special Tests for Monitoring Fetal Health. ACOG Patient Education Pamphlet AP098.
American Academy of Family Physicians. Prenatal Tests.
University of Michigan Health System. Fetal Heart Rate Monitoring. http://www.med.umich.edu/obgyn/smartmoms/labor/labor/fetalheart.htm
World Federation of Societies of Anaesthesiologists. Answers to Self Assessment Questions. World Anaesthesia.
Merck Manual. Risk Factors Before Pregnancy. http://www.merck.com/mmhe/sec22/ch258/ch258b.html