A: A contraction stress test (CST) is basically a way of making sure your baby's healthy enough to withstand the stresses of labor and delivery, since contractions during labor reduce blood flow and oxygen to your baby. Most babies can handle this just fine, but it may cause some to have a slower heart rate, which could lead to problems during delivery.
A CST is done at the end of the third trimester, and usually only in women who have high-risk pregnancies, or if results from other tests (like a nonstress test or biophysical profile) have yielded abnormal results. If you and your baby are healthy and developing on track, you probably won't need a contraction stress test. Some women should not receive a contraction stress test, including those who are at risk for preterm labor, are pregnant with twins or multiples, have had a c- section or other uterine surgery, or are at risk for placental abruption or placenta previa (conditions that make starting contractions too risky).
During a CST, your doctor will induce mild contractions while monitoring your baby's heart rate with an external fetal heart monitor. You'll be lying down and wearing two belts with sensors around your belly. One will record your baby's heart rate, the other will measure your contractions. To induce contractions, your doctor will give you low doses of oxytocin (which is the hormone your body releases to kick off labor) or may have you help things along by rubbing your nipples (which triggers a natural flow of oxytocin). The test lasts until you've had three moderately strong contractions over a 10-minute window.
The contractions should subside shortly after the test. You may have to rest until they settle down, which is why a CST can last about two hours.
You'll learn the CST results right away. A normal outcome (also called negative or reactive) means that your baby's heart rate has not slowed in response to contractions -- a sign your baby can handle labor totally fine. An abnormal, or positive, outcome means that your baby's heart rate did slow down during contractions. If this is the case, your doctor may decide to deliver your baby early by inducing labor or by performing a c-section.