OBJECTIVE: to explore pregnant women's exercise patterns across pregnancy, reported reasons for exercising or not exercising, beliefs about the safety of exercise during pregnancy and the association of those beliefs with the amount and intensity of exercise that women participated in. DESIGN: a prospective questionnaire-based approach was implemented over three pregnancy time points 8 weeks apart, with retrospective pre-pregnancy data obtained at the first time point. SETTING: participants were mailed questionnaires at 16-23-weeks pregnancy (T1), 24-31-weeks pregnancy (T2), and 32-38-weeks pregnancy (T3). PARTICIPANTS: a total of 158 pregnant women participated. MEASUREMENTS: at 16-23-weeks pregnancy women completed an Exercise Safety Beliefs Questionnaire in which they described their beliefs about the safety of low to medium exercise, high intensity exercise, gentle exercise, and weight bearing exercise. At T1, T2 and T3 reasons for exercising and not exercising were described, and participants maintained a 1-week exercise diary in which they recorded amount and intensity of physical activity. Physical symptoms experienced over time were also reported. FINDINGS: the amount and intensity of exercise decreased over the course of pregnancy, with main reasons for not exercising including feeling tired or unwell, being too busy, and, particularly in late pregnancy, exercise being uncomfortable. Some women also reported safety concerns. Safety concerns predicted amount and/ or intensity of exercise. KEY CONCLUSIONS: overall, most women had clear beliefs about what forms of exercise were safe or not safe during pregnancy. Women who rated gentle and low to medium exercise as unsafe reported engaging in less intense and fewer minutes of exercise. IMPLICATIONS FOR PRACTICE: information and discussion about ways to exercise safely, enjoyably, and comfortably should be offered to pregnant women by health professionals in early pregnancy, when safety beliefs may impact on women's exercise patterns across pregnancy, and throughout pregnancy since the most appropriate forms of exercise may need to be modified over time.