It assists the fetal ability to cope with the contraction of high-risk pregnancy and the stress of labor. It helps the physician in selecting the optimal time for delivery of the high-risk fetus. Contraction decreases the blood flow through intervillous space if the uteroplacental reserve is normal this decreases in blood flow by contraction, will not negatively affect the fetus.
If the uteroplacental reserve is diminished, there is a decrease in blood blow by contraction, which will affect the fetus, this alters the heart rate pattern by late deceleration.
CST is performed by the fetus response induced by nipple stimulation test and oxytocin using external fetal monitoring to record FHR and uterine contraction simultaneously.
PROCEDURE– The woman must empty her bladder and lie in semi fowler’s or left lateral position. Two methods for inducing the contraction for CST is-
(A) Nipple stimulation test– In this there is the release of endogenous oxytocin. This method of producing naturally occurring oxytocin avoids the risk of discomfort. At the beginning of test warm cloth is applied to breast then the woman stimulates her nipple either by rolling or gently pulling them. Initially, it is unilateral if the contraction is inadequate the women then stimulate both nipples for another 10 min. If the contraction is still inadequate (<3 contraction in 10 min.)IV oxytocin is used.
(B) Oxytocin challenge test– The administration of oxytocin to induced contraction, venipuncture is done and a very dilute solution of oxytocin control by an infusion pump is administered. The solution is prepared by dissolving 1 unit in 500 ml RL with the initial dose of 1-2 milliunit/min.(15 drops/ min.).The rate is increased at an interval of 20 min. until the contraction is frequent of at least 3 in 10 min.or lasting for at least 30 sec. then the recording is done as-
1) A negative CST is one with no late deceleration occurs with contraction as frequent as 3 in 10 min.indicate fetal well being. Can predict that the fetus will continue for another week without the intervention of delivery.
2) A positive CST is one in which there is repeated late deceleration of FSH pattern after the contraction has been passed. It indicates that the fetus may not withstand the continuation of pregnancy.