Background Knowledge of the absolute and RR of Venous Thromboembolism (VTE) in and around pregnancy is crucial in identifying when to commence and cease thromboprophylaxis in women who would benefit most from such intervention.

Methods We used electronic general practice data with details of all pregnancies in prospective primary care records of women aged 15–45 years old between 1987 and 2004. Women experiencing their first VTE event were identified, and the risks of VTE during antepartum and postpartum periods were compared with those outside pregnancy using a Poisson regression model adjusted for age and calendar period.

Findings Among 972 683 women in our cohort there were 207 327 live birth pregnancies. The overall risk of VTE compared with the risk outside pregnancy was much higher in the postpartum (HR=11.9 95% CI 9.8 to 14.5; absolute risk 228/100 000 person-years) than antepartum (HR=2.9, 2.3 to 3.7; 55/100 000 person-years). The third trimester conveyed greater risk (HR=5.3, 4.0 to 7.0) than the first (HR=1.3, 0.7 to 2.4) and second (HR=1.7, 1.0 to 2.8) trimesters. The increase in postpartum risk was predominantly in the first 6 weeks postpartum (HR=22.3), with only a small increased risk in the second 6 weeks postpartum (HR=1.8). Findings were independent of age and calendar period.

Interpretation Women are at highest risk of VTE in the third trimester of pregnancy and in the first 6 weeks postpartum. Beyond the normal age-related increase risk of VTE for all women, pregnancy does not augment further the increased RR of VTE related to age. These findings will inform the revised Royal College of Obstetricians and Gynaecologists guidelines on VTE prophylaxis.