About 70% of pregnant women experience nausea and vomiting during the first 3 months of pregnancy (the first trimester). For some, severe and persistent vomiting leads to serious dehydration that requires hospitalisation. There are several prescription drugs which are used to treat nausea and vomiting during pregnancy. Inconsistent findings from international studies suggest that women who take one of these drugs, ondansetron, during the first trimester may be more likely to have a baby with a congenital heart abnormality, cleft lip and/or palate, or other abnormalities. Ondansetron is not recommended as a first-line drug to treat nausea and vomiting in pregnancy, but its use appears to be increasing internationally and in New Zealand. The aims of our study are to examine trends in the use of ondansetron and determine whether it increases the risk of congenital abnormalities. The research will be based on anonymised existing health and pharmaceutical dispensing data.