What is a Contraction Stress Test?

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  • Written By: Allison Boelcke
  • Edited By: Bronwyn Harris
  • Last Modified Date: 21 October 2019
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A contraction stress test is a test performed during pregnancy to verify whether or not the unborn baby’s heart is strong enough to withstand labor. It uses drugs or nipple stimulation to make the uterus temporarily contract in order to replicate labor contractions. The test is typically only used if the unborn baby has had abnormal results during other pregnancy health examinations.

Uterine contractions involve the muscles of the uterus tightening and relaxing in intervals, which helps to eventually push the fetus out of the body during labor. During these intervals, the blood flow to the fetus is slightly reduced. Since the blood contains the fetus’s oxygen supply, the fetus is subsiding on reduced oxygen during delivery. If the fetus has a healthy heart, it can typically survive with the temporarily limited oxygen.

A contraction stress test is a means of simulating uterine contractions under a doctor’s supervision before labor occurs. If a doctor performs an ultrasound and discovers the unborn baby has a weakened heartbeat, he or she may recommend the contraction stress test. It is generally done during the last trimester of pregnancy, often around 34 weeks of gestation.


Oxytocin is a hormone that naturally occurs in the brain of both males and females and is responsible for feelings of pleasure during intimacy or bonding, ranging from hugging to orgasms. This hormone is discharged in higher amounts during the later stages of labor. A doctor will inject a pregnant woman with synthetic oxytocin to begin the stress test while monitoring the heartbeat of the fetus. The body will think it is in labor, which will cause the uterus to begin contracting. Rubbing the nipples may also possibly release the hormone naturally to start the contractions if the hormone does not cause contractions; however, it can be more difficult to control the amount of the hormone using nipple stimulation. If too much of the hormone occurs in the body during the test, it can start labor prematurely.

Once the uterus contracts in regular intervals of three times every ten minutes, the doctor will monitor the heartbeat of the fetus until the contractions stop naturally. The test results are considered safe and normal if the heartbeat does not slow down during the contractions. If the heartbeat slows down and remains slow during the contractions, the fetus may not be able to survive with the lowered amounts of oxygen during labor. A fetus with abnormal contraction stress test results will generally need to be delivered with a cesarean section, a surgery in which a doctor makes in incision in the woman’s abdomen and removes the baby.


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