Preterm nonstress testing: 10-beat compared with 15-beat criteria

To evaluate the relation of different acceleration criteria to perinatal outcome for interpretation of nonstress test (NST) at 32 weeks of gestation or earlier.

Nonstress tests within 1 week of delivery for women at 32 weeks of gestation or earlier were classified as reactive by 10-beat criteria, reactive by 15-beat criteria, or nonreactive. In women with three NSTs within 1 week of delivery, patterns were categorized into decreasing, increasing, irregular, or consistent reactivity. Perinatal outcomes included perinatal death, intensive delivery room resuscitation, 5-minute Apgar score less than 7, mechanical ventilation, necrotizing enterocolitis, intraventricular hemorrhage, and hypotension. Analysis included χ and logistic regression.

Seven hundred fifty-one NSTs from 488 women were assessed. Of the final NSTs before delivery, 105 (21.5%) were nonreactive, 151 (30.9%) were reactive by 10-beat criteria, 232 (47.5%) were reactive by 15-beat criteria, and 274 (56.1%) had decelerations. Unadjusted data consistently associated degree of reactivity with outcome and also associated gestational age with degree of reactivity. After adjustment for gestational age and birth weight, there was no association between NST criterion (or decelerations) and outcomes except between nonreactivity and perinatal death (odds ratio 6.1, 95% confidence interval 2.3-16.5). For 97 women who underwent three tests, all patterns were common (range 16.5-37.1%) and none was significantly associated with any outcome.

In preterm pregnancies, the criterion used for NST interpretation is much less important than the gestational age at delivery. There was no appreciable difference between the 10-beat criteria and 15-beat criteria in predicting outcome. When tested repeatedly, fetuses frequently had varying degrees of reactivity without relation to outcome.