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Functional tests for fetal assessment
Last reviewed: 08.07.2025
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The most common tests in obstetric practice are non-stress and oxytocin tests.
The oxytocin test is simple, harmless and to a certain extent physiological, i.e. it is a test that imitates normal childbirth.
There are two main conditions for performing the oxytocin test (OT):
- oxytocin is administered intravenously with a progressive increase in dose from 1 to 4 mU/min;
- The test is stopped when late decelerations appear.
All other parameters may vary - the duration of the test, the number, frequency and intensity of uterine contractions, the recording technique. When performing the test, the pregnant woman or the woman in labor is placed on her side to avoid the Pozeiro effect. The most important for the clinician is a positive oxytocin test with the appearance of late decelerations.
Some authors use the maternal stress test.with physical work and a corresponding decrease in uterine blood flow, as well as a step test.
Also of interest is the test with low O2 content in the mixture given to the mother to inhale, causing hypoxia. This test is good for monitoring placental function.
The atropine test is based on the fact that atropine, passing to the fetus through the placenta, leads to tachycardia of 20-35 beats/min, which occurs 10 minutes after the injection of atropine at a dose of 1.5-2 mg in 5 ml of 40% glucose solution and lasts 40-70 minutes.
The non-stress test (NST) is currently the most common and most valuable method for assessing the condition of the fetus. The duration of the test should be at least 30 minutes. However, some authors, based on the conclusion that the fetus should be in a state of rest for 50-75 minutes, suggested that 120 minutes are needed to conduct a non-stress test.
The use of a non-stress test in low-risk pregnancies has shown that the incidence of fetal hypoxia in groups with an areactive type of heart rate curve (no decelerations or accelerations of the rhythm during the observation period) or with a deceleration of the rhythm was 33%, while with other types of fetal heart rate curves (reactive, hyporeactive and reactive group with a deceleration of the rhythm) the incidence of hypoxia varied from 0 to 7.7%. The test is considered reactive if there are 5 accelerations in response to fetal movements during any 20-minute interval. The reactive non-stress test gives a favorable prognosis in pregnancies in 98.5%, and the areactive non-stress test gives an unfavorable prognosis in 85.7% of pregnant women. However, it is important to emphasize that the non-stress test is an indicator based on the results of which it is possible to judge the condition of the fetus only at the time of the test. The non-stress test cannot be used for long-term prognosis.
Most researchers believe that the normal number of heartbeat accelerations should be more than 3 per 30 minutes of recording, each acceleration period should be more than 30 seconds, and their number should be more than 17 beats/min. The data from the reactive non-stress test and the oxytocin test coincide completely, and therefore the oxytocin test is unnecessary for the reactive non-stress test. Both tests are often uninformative for assessing the risk of intrauterine fetal death.
False negative results in a non-stress test are most often observed in cases of placental abruption, congenital malformations, and umbilical cord pathology.
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