Fetal Cardiac Function Shifts Following Tocolytic Therapy With Nifedipine or Magnesium Sulfate: A Comparative Doppler-Based Prospective Assessment

Background: To compare short-term changes in fetal cardiac function following tocolytic therapy with nifedipine or magnesium sulfate (MgSO4) in pregnancies complicated by threatened preterm labor. Methods: This prospective observational study included 316 singleton pregnancies between 32–34 weeks of gestation presenting with threatened preterm labor at a tertiary perinatology clinic. Women received either oral nifedipine (n = 280) as first-line tocolysis or intravenous MgSO4 (n = 36) when nifedipine and indomethacin was contraindicated. Fetal echocardiography was performed within 2 h before initiation and 2–4 h after cessation of tocolysis. Mitral and tricuspid E and A velocities, E/A ratios, left and right myocardial performance indices (MPI), and fetal heart rate (FHR) were recorded. Pre-and post-treatment changes, as well as between-group differences, were analyzed. Receiver operating characteristic (ROC) curves were constructed to assess the discriminative performance of cardiac indices. Results: A total of 316 pregnant women were included in the analysis, with 280 receiving nifedipine and 36 receiving MgSO4. Baseline demographic and clinical characteristics were comparable between groups, except for a slightly higher gestational age and longer duration of tocolysis in the MgSO4 group. In the nifedipine group, post-treatment mitral and tricuspid E/A ratios increased significantly, accompanied by a small but statistically significant decrease in left ventricular MPI and a significant decrease in FHR. In the MgSO4 group, E/A ratios increased significantly after treatment, whereas individual E and A wave velocities and MPI showed no significant changes. In both groups, FHR decreased significantly after treatment. ROC analyses demonstrated limited to moderate discriminative performance of individual cardiac parameters in distinguishing pre- and post-tocolysis states. Conclusions: Both nifedipine and MgSO4 were associated with short-term alterations in fetal diastolic filling parameters following tocolysis. Nifedipine additionally showed a small but statistically significant decrease in left ventricular myocardial performance index (MPI). All observed changes remained within physiologically acceptable ranges for late gestation, supporting the fetal cardiac safety of both agents when used for tocolysis. Clinical Trial Registration: The study has been registered on https://clinicaltrials.gov/ (registration number: NCT06904534; registration link: https://clinicaltrials.gov/study/NCT06904534?cond=NCT06904534&viewType=Card&rank=1).