Contraction Stress Test

Fetal movements translate to fetal well-being. Of course there are times when the fetus is not active and this does not necessarily imply that the fetus is in trouble. For instance it is normal for the fetus to move less towards the end of the term because there is less room for movement. There are also situations when due to extra amniotic fluid the mother is not able to feel the movements. To be on the safe side, contact your care provider if you notice movements have decreased considerably. Tests will be carried out to analyze the baby's well being. One such test is the CST or contraction stress test.

What is CST..

CST is based on the observation that during contractions, blood flow to the placenta lessens temporarily. An evaluation is done on how the fetus handles this stress.

Normally fetal heart rate is not affected by contractions

In actual labor after contraction begins, if the fetal heart beat slows down, it indicates that the fetal is not able to tolerate the decreased blood flow resulting from the contraction.

These decreases are called late decelerations .

If the placenta is not working to capacity or the baby has some problem, Contraction can decrease the oxygen flow and cause the heart rate to drop.

CST is used if nonstress test showed no change in the fetal heart rate when the fetal moved

The logic behind the test is to challenge the fetus with contractions for brief periods to see if the fetal heart demonstrated decreases. Positive test usually indicates that the fetus needs to be delivered

The Procedure itself..

Two fetal monitors will be strapped to the woman's abdomen to record fetal heart rate. She will lie on her left side

It is usually conducted if the pregnant woman has had problematic pregnancies in the past or has medical problems in her current pregnancy.

CST is usually performed if nonstress test wasn't reassuring in the last few weeks of pregnancy

One monitor will pick up uterine contractions and the other picks up fetal heart beat. Both will readings will record on graph paper

The heart beat will form a line at the top and the maternal contractions will form wavelike lines at the bottom. Both lines will be matched to determine the significance of any decelerations

Contractions are either induced with oxytocin, a hormone usually used to induce labor. It is given through an IV line. Before this the baseline heart beat will be recorded for about 20 minutes max

In place of oxytocin, nipple stimulation can also be used to induce natural oxytocin in the system. The mother will be asked to rub her nipple for about 15 minutes until adequate contractions have been achieved.

The plus side of nipple stimulation is contractions are induced more quickly and an IV is not required

Either way the responses are obtained and recorded

Three contractions must take place in the 10 minute duration and each contraction must last for about 40 seconds. This is regarded as the approximate stress the fetus will endure during labor

The test can last for a good 2 hours

Result Interpretations..

The possible interpretations of CST are:

Negative - this is the desired result. It implies that no late decelerations are present in the presence of adequate contractions, the placenta is functioning properly and the fetus will be doing fine for at least a week. In high risk pregnancies CST will have to be performed more than once a week

Positive late decelerations are obvious in the presence of adequate contractions. Delivery of baby follows a positive result either by induction of labor or C-section, whichever appropriate


Suspicious the late decelerations are not consistent. A suspicious CST requires a repeat of test usually within the same day or 24 hours

Hyperstimulation CST showing contractions are happening too frequently (5 or more in 10 minutes) or its lasting too long (90 seconds or more). Hyperstimulated test is considered negative if decelerations are absent; it can be considered invalid if decelerations do occur. CST should be repeated in 24 hours

Unsatisfactory fetal heart rate cannot be traced or the desired contractions (3 in 10 minutes) cannot be achieved. CST should be repeated in 24 hours

Fortunately there are very, very few incidence of false negative' results. Essentially a false negative result will show the fetus is faring well when in actual fact it is in distress