It monitor the fetal heart rate in response to fetal movement, in order to assess the fetal well being.In this there is no use of external stimuli (oxytocin) while conducting the the test so it is useful in women for whom CST might be contraindicated like placenta praevia .
PREPARATION-The woman must empty her bladder & lie in semi fowler’s or left lateral position. FHR is then maintained for approximate 20-30 min. It is important that fetus not be in sleep state during entire procedure. An abdominal palpation can be used to arouse the fetus.
During the test the women presses the button whenever she feels movements which produce mark on the monitor strip so fetal movement and fetal heart rate can be corrected. interpretation of NST is based on pattern of fetal heart rate, reactivity of fetal movement.
Normally the fetal heart rate decreases and variability increases with gestational age probably in relation to development of CNS in fetus, normally has a transient accelerated heart rate with average baseline variability when moving or to extent stimuli such as abdominal palpation.Abnormal fetal heart rate reactivity response to fetal movements is evidenced by persistent reduction in baseline variability with an absence of heart rate acceleration .This pattern may indicate a distressed fetus and may occur when fetus CNS is depressed by drugs ,hypoxia or acidosis.
REACTIVE TEST– In which normal fetus reactivity pattern is demonstrated . This is evidenced by fetus heart rate is accelerated by 15 bpm and baseline lasting for 15-30 sec. in association with fetal development 2 or more occurrences of this acceleration pattern within a 10 min. period or 5 or more acceleration in 20 min period considered normal.
NON REACTIVE TEST–In which there is persistent decreased variability with an absence of acceleration of fetal heart rate, in response to fetal heart rate acceleration of less than 15 bpm above the baseline variability or lasting less than 15 sec. in association with fetal movements.
A reactive test indicates fetal well being and predicts a good outcomes if birth were to occur within 1 week .If a test is non reactive further assessment of fetal status should be initiated to aid in determining fetal well being. If the test is suspicious it should be repeated in 24-48 hrs. The NST can be repeated or continued at any desired frequency without concern for adverse effect to the fetus.