Braxton-Hicks contractions can alter uteroplacental perfusion

Color flow imaging was used to study the effect of spontaneous low-amplitude uterine contractions (Braxton-Hicks) on flow velocity waveforms obtained from the main uterine artery in 13 women between 26 and 34 weeks' gestation. Eight women had abnormal waveforms, as defined by the presence of an early diastolic notch, and five women had normal waveforms. In the former group, three women with chronic hypertension were started on the calcium antagonist nifedipine during this study. Contractions, which were monitored by external tocography, occurred more frequently in the group with abnormal waveforms and were temporally associated with an increase in impedance to blood flow. This effect occurred to the same degree in both normal and abnormal groups, but in the abnormal group the already impaired blood flow to the intervillous space was further diminished, resulting in absence of forward flow in diastole. Such acute reductions of flow in response to frequent, impalpable uterine contractions may cause a prolonged and silent insult to uteroplacental perfusion, which could hasten fetal compromise. This effect appeared to be abolished in patients on nifedipine, suggesting a possible therapeutic role for this drug in uteroplacental insufficiency.