Effect of oral calcium carbonate on uterine contractility: a pilot randomised controlled trial

Ineffective uterine contractions contribute to labour dystocia and are a leading indication for primary caesarean delivery. Although oxytocin is the standard therapy for augmentation, prolonged exposure may result in receptor desensitisation and reduced effectiveness. Calcium is essential for myometrial contraction; however, systemic calcium homeostasis is tightly regulated, and it is unclear whether oral calcium supplementation can meaningfully influence uterine activity. This study aimed to evaluate the effect of oral calcium carbonate on uterine contractility.

We conducted a single-centre randomised controlled pilot trial at a tertiary care teaching hospital (ClinicalTrials.gov: NCT07056062). Term patients with singleton foetus in cephalic presentation and an intrauterine pressure catheter in place were randomised 1:1 to receive a single 2,000 mg oral dose of calcium carbonate or no intervention. Uterine activity was measured using Montevideo units (MVUs) at baseline and at 30-minute intervals for two hours. Secondary outcomes included contraction frequency, peak contraction pressure, labour duration, mode of delivery, oxytocin dose, and postpartum haemorrhage. Oxytocin infusion rates were held constant during the observation period.

Eighty-nine patients were analysed (45 control; 44 intervention) with similar baseline characteristics. No statistically significant difference in baseline MVUs was observed between groups (p = 0.1825). Although absolute MVUs were higher in the intervention group at 30 minutes (p = 0.0500), this difference was not sustained at subsequent time points and was absent in change-from-baseline analyses, suggesting no clinically meaningful treatment effect. No statistically significant differences were identified in secondary outcomes.

Oral calcium carbonate did not result in a statistically significant improvement in uterine contractility or clinical outcomes. These findings likely reflect the limited physiologic effect of oral calcium on serum calcium levels. Oral calcium carbonate appears unlikely to be an effective intervention for labour augmentation; future research should focus on strategies with more controllable mechanisms.