OBJECTIVES: To investigate whether age at menarche is related to maternal blood pressure in pregnancy and, if so, whether obesity and insulin resistance can modify the associations. STUDY DESIGN: Analysis of data collected from 438 pregnant women from the longitudinal and prospective Cambridge Baby Growth Study. MAIN OUTCOME: Testing associations between questionnaire-derived age at menarche and blood pressure measurements in pregnancy collected from hospital notes, and investigating whether any associations were altered by maternal pre-pregnancy body mass index (BMI) and insulin resistance. MEASURES: Mean arterial blood pressure at four time points across pregnancy, age at menarche, (Homeostasis Model Assessment) insulin resistance around week 28 of pregnancy. RESULTS: For each increased year in age at menarche there was a drop in mean arterial blood pressure (mmHg) of 0.6 at 11.9 weeks, 0.9 at 31.4 and 37.0 weeks, and 0.4 at 38.8 weeks (a maximal difference of over 7 mmHg across extremes of AAM). Each association was attenuated by both maternal pre-pregnancy BMI and insulin resistance. CONCLUSIONS: Age at menarche is negatively associated with future blood pressure in pregnancy, so those with the earliest age at menarche have the highest blood pressures. Either these associations may be mediated by links between age at menarche and obesity/insulin resistance, or there may be a confounder (e.g. systemic inflammation) that links age at menarche to each of them.