A prospective analysis of the influence of gestational age on the baseline fetal heart rate and reactivity in a low-risk population.

The nonstress test is of recognized value for the antepartum assessment of fetal well-being. Less well defined, however, is the relationship between baseline fetal heart rate, reactivity, and the gestational age of the fetus. A prospective analysis of the nonstress test in a preterm, low-risk population was begun. Weekly tests were begun as early as 23 weeks and continued to term. A test was considered reactive if there were two or more fetal heart rate accelerations within a 20-minute period. A total of 128 nonstress tests were performed on 25 low-risk patients. Of these tests, 84 (64%) were considered reactive. With advancing gestational age, a progressive increase in the incidence of reactive tests was observed. However, no significant difference in baseline fetal heart rate could be identified between term and preterm fetuses. In addition, the frequency and amount of accelerations and decelerations were determined. Decelerations were not noted more frequently in the preterm population. Conversely, accelerations occurred more frequently with advancing gestational age. The pregnancy outcome was excellent for all study patients. It is concluded, therefore, that with advancing gestational age more reactive nonstress tests are seen. Baseline fetal heart rate and the frequency of decelerations appear to be unaffected by advancing gestational age. Finally, although the numbers are small, the nonstress test appears to be a suitable technique for assessing fetal well-being in the preterm fetus.