Similar to the non-stress test (NST), the contraction stress test (CST) is a measure of overall well being. It is able to indicate problems earlier than a NST, but takes more time to do and costs more.
While the NST is looking for reactivity, the CST is looking for reactions that are evidence of a problem. These reactions are called late decelerations and they are believed to happen when the baby is not getting enough oxygen. The lack of oxygen makes is harder for the baby to recover from the contraction.
There are two ways this can be done, with artificial oxytocin or with nipple stimulation. Artificial oxytocin is injected via an IV and the amount will be increased every 15 to 30 minutes until you have three good contractions in 10 minutes. Nipple stimulation involves massaging your own nipple (either one nipple for 10 minutes straight or massaging one nipple for two minutes, resting for five minutes then repeating the cycle as necessary) until adequate contractions are achieved.
What you need to know
There is a low false negative rate which means baby’s who pass the test are healthy. However there is a 30% false positive rate which means 30% of the babies who test as having late deceleration are fine. Because of this, a positive test (one in which late decelerations are seen) is usually followed up with more accurate diagnostic testing to verify the results.
You will be asked to sit in a recliner or to lye on your side. An electronic fetal monitor will be placed on your abdomen to read your baby’s heart rate and to measure contractions. After ten to twenty minutes to establish a baseline heart rate, you will begin the process of stimulating contractions. How long this takes depends on how long it takes to establish an adequate contraction pattern for reading.
Devoe, L.D. (2008). Antenatal fetal assessment: contraction stress test, nonstress test, vibroacoustic stimulation, amniotic fluid volume, biophysical profile, and modified biophysical profile–an overview. Seminars in Perinatology 32(4):247.