The effects of antenatal betamethasone administration on fetal heart rate and behaviour depend on gestational age

We previously reported decreases in fetal heart rate (FHR) variability and body and breathing movements after maternal betamethasone administration. We now test the hypothesis that fetal responsiveness to betamethasone depends on the gestational age at which glucocorticoid therapy is started.

1-h recordings of FHR (n=350) and fetal movements (n=310) made during a 5-day period (days 0-4) were available for analysis. The recordings had been obtained from 63 pregnant women at high risk for preterm delivery who received betamethasone (two doses of 12 mg 24 h apart) between 26 and 34 weeks' gestational age (wGA). The response to betamethasone, i.e. the direction and magnitude of change in FHR and movement parameters compared with baseline (day 0), was studied in relation to gestational age at drug administration.

Fetuses exposed to betamethasone at 29-34 wGA showed a decrease in FHR on day 1 (indicative of baroreceptor reflex), and reduced breathing activity and prolonged episodes of quiescence with a concomitant decrease in body movements on days 1 and 2. However, these changes were not observed if betamethasone administration occurred at 26-28 wGA. Betamethasone-induced reductions in FHR variability were similar in young and older fetuses.

Age-related differential responsiveness to betamethasone was found for all studied fetal processes (body and breathing movements, FHR, and quiescence), except FHR variability. Our results suggest ontogenic changes in the mechanisms presumed to underlie these processes (glucocorticoid receptor (GR) maturation, cardiovascular and neuro-endocrine development).