Objective To assess adherence to anti-hypertensive medication by pregnant women and to understand the factors associated with adherence or lack thereof. Design Observational study. Setting Antenatal clinics. Population 100 pregnant women with either chronic hypertension or gestational hypertension who were being treated using at least one anti-hypertensive medication. Methods Participants were prospectively recruited through two Australian maternity hospitals over a 10-month period. A 23-item questionnaire, which incorporated a validated adherence scale, was administered to all participants. Data from clinic records were extracted as needed. Main Outcome Measures Self-reported adherence, BP control and reasons for nonadherence. Results Participants (mean age 33 [±4.9] years; mean gestation 29 (±7) weeks) had a median blood pressure (BP) of 130/80mmHg (IQR: 16/15). Sixty-five women had chronic hypertension, of whom 13 were diagnosed during the pregnancy, before 20 weeks gestation. Thirty-five women had gestational hypertension. Ninety-two per cent of participants had sub-optimal adherence. There were no significant differences in adherence scores between participants with chronic hypertension and their counterparts. The main reasons for sub-optimal adherence were: intentionally putting up with medical problems before taking any action, confusion about the medication, and making changes to the recommended medication regimen to suit lifestyle. Conclusions Nine out of 10 pregnant women using anti-hypertensives self-reported some degree of suboptimal adherence, intentionally and/or unintentionally. Health professionals, including pharmacists, general practitioners and obstetricians, have a role in promoting optimal adherence. Funding No funding received. Keywords Adherence, anti-hypertensives, pregnancy, gestational hypertension, chronic hypertension.