The combined effect of betamethasone and ritodrine on the middle cerebral artery in low risk third trimester pregnancies

Aims: To evaluate the effect of antenatal betamethasone and ritodrine in third trimester low risk singleton pregnancies by Doppler technology. Subjects and methods: Eighty-four third trimester pregnant women who received a full course of betamethasone and delivering uneventfully were recruited. The Doppler examination included the assessment of the pulsatility index (PI) of the umbilical artery (UA PI) and the middle cerebral artery (MCA PI) prior to treatment (baseline), and 48, 72 and 96 h after the second dose of betamethasone. Results: No significant difference was found in UA PI and UA/MCA values following betamethasone therapy. In contrast, MCA PI decreased significantly 48 h from the last injection of betamethasone in the whole study group (P < 0.001), and returned to basal values at 96 h. We also found that MCA PI was reduced significantly at 48 h in the subgroup under tocolysis (n = 41) and in the subgroup not receiving tocolysis (n = 43). We compared MCA PI values for both subgroups in the four timings, and found a non-significant difference comparing baseline and 96 h values. However, when comparing MCA PI values after 48 and 72 h, significantly lower differences in PI values in both subgroups were found. Conclusion: In low risk pregnancies, betamethasone therapy in the third trimester is related to a significant but transient reduction of MCA PI, which is more pronounced during tocolytic therapy. Although the physiological basis of this effect is currently unclear, it could be related to the local regulation of intracerebral blood flow.