This study investigated strategies for ascertaining pre‐eclampsia, pregnancy hypertension and gestational diabetes mellitus from birth records and/or hospital discharge data. The results showed that ascertaining these conditions from a data set that linked birth records to the corresponding maternal hospital record for birth was sufficient for health outcomes research. Antenatal hospital records provided few extra cases and may be necessary only for the ascertainment when a very accurate estimate of the prevalence is required.