Background Aspiration pneumonitis is a syndrome resulting from the inhalation of gastric contents. The incidence in obstetric anaesthesia has fallen, largely due to improved anaesthetic techniques and the increased use of regional anaesthesia at caesarean section. However, aspiration pneumonitis is still a cause of maternal morbidity and mortality, and it is important to use effective prophylaxis. Objectives To determine whether interventions given prior to caesarean section reduce the risk of aspiration pneumonitis in women with an uncomplicated pregnancy. Search methods We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (April 2009). Selection criteria Randomised controlled trials were included. Quasi-randomised trials were excluded. Data collection and analysis Authors independently assessed the studies for inclusion, assessed risk of bias and carried out data extraction. Data entry was checked. Main results Twenty-two studies, involving 2658 women, are included, all having a caesarean section under general anaesthesia. The studies covered a number of comparisons, but were mostly small and of unclear or poor quality. When compared to no treatment or placebo, there was a significant reduction in the risk of intragastric pH <2.5 with antacids (risk ratio (RR) 0.17, 95% confidence interval (CI) 0.09 to 0.32, two studies, 108 women), H2 antagonists (RR 0.09, 95% CI 0.05 to 0.18, two studies, 170 women) and proton pump antagonists (RR 0.26, 95% CI 0.14 to 0.46, one study 80 women). H2 antagonists were associated with a reduced the risk of intragastric pH <2.5 at intubation when compared with proton pump antagonists (RR 0.39, 95% CI 0.16 to 0.97, one study, 120 women), but compared with antacids the findings were unclear. The combined use of 'antacids plus H2 antagonists' was associated with a significant reduction in the risk of intragastric pH <2.5 at intubation when compared with placebo (RR 0.02, 95% CI 0.00 to 0.15, one study, 89 women) or compared with antacids alone (RR 0.12, 95% CI 0.02 to 0.92, one study, 119 women). Authors' conclusions The quality of the evidence was poor, but the findings suggest that the combination of antacids plus H2 antagonists was more effective than no intervention, and superior to antacids alone in preventing low gastric pH. However, none of the studies assessed potential adverse effects or substantive clinical outcomes. These findings are relevant for all women undergoing caesarean section under general anaesthesia.