OBJECTIVES: To test the hypothesis that bacterial infections in pregnancy are related to maternal blood pressure. STUDY DESIGN: Bacterial infection was assessed using antibiotic usage as a surrogate and its association with blood pressure in pregnancy tested in the Cambridge Baby Growth Study. MAIN OUTCOME MEASURES: Antibiotic usage in pregnancy was self-reported in questionnaires. Blood pressure measurements at four time points in pregnancy were collected from the hospital notes of 622 women. RESULTS: Using all the available blood pressure readings (adjusted for weeks gestation) antibiotic usage was associated with a higher mean arterial blood pressure across pregnancy: antibiotics used 85 (84, 87) mmHg vs. no antibiotics used 83 (83, 84) mmHg (β = 2.3 (0.6, 4.0) mmHg, p = 9.6 × 10−3, from 621 individuals). Further analysis revealed that antibiotic usage was associated with diastolic (β = 2.3 (0.6, 4.0) mmHg; p = 7.0 × 10−3) more than systolic blood pressure (β = 1.4 (−0.9, 3.7) mmHg; p = 0.2). The effect size associated with antibiotic usage appeared to rise slightly after the first trimester. CONCLUSIONS: Bacterial infection in pregnancy, as assessed by self-reported antibiotic usage, is associated with small rises in blood pressure.