Assessment of fetal well-being in methadone-maintained pregnancies: abnormal nonstress tests

To investigate parameters of fetal well-being (characteristics of nonstress test, NST, and antepartum fetal heart rate, FHR, patterns) and selected neonatal outcomes in pregnant women on methadone maintenance.

A matched case-control study of methadone-treated women receiving prenatal and intrapartum care at a Bronx municipal hospital during 1992-1994. 102 NSTs obtained from 24 methadone-treated women after 35 weeks of pregnancy were compared to 96 NSTs from a control group (n = 24), matched for maternal age, parity, and gestational age. All NSTs were evaluated for general characteristics including time interval between initiation and achievement of reactive NST (2 accelerations > or = 15 bpm lasting for at least 15 s in a 20-min period), baseline, amplitude of fluctuation, frequency of fluctuation, accelerations and decelerations. The scoring system described by Lyons et al. was used for all NSTs. All nonreactive NSTs were followed with biophysical profile tests.

The frequency of nonreactive NSTs was significantly higher for methadone-maintained women compared to controls (19.6 vs. 4.2%; p < 0.01). The average length of time to achieve reactive NST was significantly (p = 0.0016) longer for the methadone-treated group when compared to controls (35.50 +/- 20.96 vs. 14.85 +/- 9.03 min). The total score (Lyons et al.) was significantly lower (p < 0.0007) for the methadone-treated group compared to controls. Mean birth weight, Apgar scores at 1 and 5 min, meconium, and umbilical cord artery pH were not significantly different for methadone-exposed neonates compared to controls.

Methadone-maintained pregnancies are significantly associated with a higher incidence of nonreactive NSTs, longer intervals to achieve reactive NSTs and lower NST scores compared to controls. This may reflect an altered response in fetal central nervous system neurotransmitters and changes in fetal behavior induced by methadone.