OBJECTIVES: To compare the completeness of maternity data from a primary care database, The Health Improvement Network (THIN), and a secondary care database, Hospital Episodes Statistics (HES). METHODS: From 1998-2009, 2,649 pregnancies were identified from one practice in THIN.  A group of 15,166 patients in the same practice was linked to secondary care data from HES and 3,285 pregnancies were identified from this population. The completeness of recording for maternity variables within each dataset was compared. RESULTS: Maternal age was recorded for all pregnancies in both datasets and a measure of social deprivation was recorded for over 95% of pregnancies in THIN and HES. Ethnicity recording in THIN was inconsistent with 56.4% missing or unknown, whilst in HES it was recorded for 75.1% of pregnancies. In THIN, 0.2% of pregnancies had data on the number of babies compared to 94% of pregnancies in HES. In both datasets, less than 25% of pregnancies had the number of previous pregnancies recorded. In HES, over 60% of pregnancies contained data on birth weight, sex of the baby and mode of delivery compared to fewer than 7% of pregnancies in THIN. Gestational age at birth was recorded for less than half of pregnancies in both THIN and HES. CONCLUSIONS: These maternity data are better recorded in HES than in THIN, suggesting possible benefits from linking the two data sources – primary care provides clinical information e.g. medical conditions and prescriptions, whilst secondary care provides data on pregnancy and birth outcomes. These findings are currently limited to one practice. Future work includes examining the baby’s records in THIN and HES for further data, assessing other variables, such as non-live birth outcomes, and expanding the analysis to a larger dataset of THIN-HES linked practices.