Background The perimenopausal and postmenopausal periods are times of pronounced physiological change in body mass index (BMI), physical activity and energy intake. Understanding these changes in middle age could contribute to formation of potential public health targets.

Method A longitudinal cohort of 5119 perimenopausal women from the Aberdeen Prospective Osteoporosis Screening Study (APOSS) recruited between 1990 and 1994, with follow-up visits at 1997–1999 and 2009–2011. At each visit, participants were weighed, measured and completed socioeconomic and demographic questionnaires. Participants at the first visit were asked to recall body weights at 20, 30 and 40 years of age. We assessed trends in BMI, physical activity and energy intake across and within visits.

Results Over 2 decades, obesity prevalence doubled from 14% to 28% of the participants, with 69% of participants being categorised as overweight or obese. Greater than 70% of participants gained >5% of their baseline BMI with weight gain occurring across all weight categories. Energy intake and physical activity levels (PALs) did not change during the 2 decades after menopause (p trend=0.06 and 0.11, respectively), but, within the second visit, energy intake increased concomitantly with a decrease in physical activity across increasing quartiles of BMI (p trend <0.001 for all).

Conclusions Overweight and obesity increased by over 50% over the course of 20 years. Weight gain occurred across the adult life course regardless of starting weight. The marked increase in dietary intake and decrease in PALs in middle age suggest a potential critical period for intervention to curb excess weight gain.