OBJECTIVE: To evaluate platelet function in mothers and fetuses/neonates exposed to magnesium sulfate intrapartum. METHODS: This was a prospective cohort study of mothers and fetuses/neonates exposed to magnesium sulfate intrapartum compared to mothers and fetuses/neonates not exposed. Platelet aggregometry was performed in duplicate on specimens from subjects using two different agonists, ADP and collagen, on a whole blood impedance aggregometer. RESULTS: Specimens were processed from 11 mothers and 10 fetuses/neonates exposed to magnesium and 12 mothers and fetuses/neonates, not exposed. There was no difference in platelet aggregation between the specimens from the mothers who were exposed and mothers not exposed. In the specimens from the fetuses/neonates, there was no difference using collagen as an agonist, but using ADP as an agonist, there was a remarkable difference - 0.0 ohms in the exposed group vs. 3.0 ohms in the unexposed group (p < 0.01). CONCLUSIONS: In-utero magnesium sulfate exposure significantly suppresses fetal platelet function.