Gut microbiota is linked with health, including obesity, in the general population. It is unknown whether adiposity at postpartum is influenced by gut microbiota already during pregnancy. We investigated the association between the gut microbiota's composition and predicted function by metagenomics during pregnancy and the women's adiposity (body mass index [BMI], waist-to-hip ratio [WHR], body fat%) assessed at 1-, 2-, and 5–6-years' postpartum in 257 women with overweight or obesity based on prepregnancy BMI values. Body fat% at 1-year, but not at 2- or 5–6-years' postpartum, was associated inversely with α-diversity during pregnancy. Bacterial species GGB3034 SGB4030 (family Erysipelotrichaceae) was higher in women with normal weight than those in women with obesity at 1-year postpartum (q = 0.02), other species being borderline statistically significant (q < 0.25). High WHR and body fat% at 1-year postpartum were associated with two species (q < 0.25). Considering predicted functions of bacteria, an association was detected for BMI, WHR, and body fat%, e.g., body fat% and glycogen biosynthesis I (q < 0.25). Gut microbiota during pregnancy predicted the BMI and body fat% at 1-year postpartum (ROC > 0.50, p < 0.02). Postpartum adiposity was associated with several species and α-diversity. Gut microbiota during pregnancy may be involved in the persistence of obesity and its comorbidities after pregnancy.