OBJECTIVES: Understanding patterns of maternity care requires knowing which women have given birth previously, but this information is typically unavailable in administrative hospital data sets. We assessed how well parity can be derived using linked historical records. STUDY DESIGN AND SETTING: Using Hospital Episode Statistics data, we identified records of women who gave birth between April 2009 and March 2010 in English National Health Service hospitals. The parity coded in these records was compared with an estimate derived from deliveries identified in previous hospital admissions between April 2000 and March 2009. RESULTS: We identified 358,849 eligible deliveries with complete parity data in the 2009-10 birth records. The historical data classified 168,041 women as multiparous; of whom, 98% were coded as multiparous in their birth record. Among 190,798 women classified as primiparous using historical data, 72% were coded as primiparous in their birth record. The proportion of accurate predictions about primiparous status from historical data varied with age, ranging from 89% for 15-18 year olds to 50% for women aged more than 35 years. CONCLUSION: Historical records in administrative hospital data sets give accurate information on multiparous status of women. There is some misclassification of primiparous status, and error rates differ among subgroups of women.