Pregnancy can alter a woman’s weight gain trajectory across the life course and contribute to the development of obesity through retention of weight gained during pregnancy. This study aimed to identify modifiable determinants associated with postpartum weight retention (PPWR; calculated by the difference in pre-pregnancy and 6 month postpartum weight) in 667 women with obesity from the UPBEAT study. We examined the relationship between PPWR and reported glycaemic load, energy intake, and smoking status in pregnancy, excessive gestational weight gain (GWG), mode of delivery, self-reported postpartum physical activity (low, moderate, and high), and mode of infant feeding (breast, formula, and mixed). At the 6 month visit, 48% (n = 320) of women were at or above pre-pregnancy weight. Overall, PPWR was negative (−0.06 kg (−42.0, 40.4)). Breastfeeding for ≥4 months, moderate or high levels of physical activity, and GWG ≤9 kg were associated with negative PPWR. These three determinants were combined to provide a modifiable factor score (range 0–3); for each added variable, a further reduction in PPWR of 3.0 kg (95% confidence interval 3.76, 2.25) occurred compared to women with no modifiable factors. This study identified three additive determinants of PPWR loss. These provide modifiable targets during pregnancy and the postnatal period to enable women with obesity to return to their pre-pregnancy weight.