Objective The study objective was to examine maternal and infant outcomes for obese GDM women who lost weight or gained 0-5 kg during pregnancy. Design A 7-year retrospective study of pregnancy outcomes for obese GDM women. Setting The ATLANTIC DIP consists of 5 antenatal centres along the Irish Atlantic seaboard. Population A total of 754 women met the inclusion criteria. Methods Women were stratified into 3 distinct groups according to their weight gain status: lost weight or gained less than 5 kg (Group 1, n=237 (31.4%)), gained 5-9kg (Group 2, n=77 (10.2%)) or gained >9kg (Group 3 n=440 (58.4%)). The groups were further subdivided according to treatment modality: diet alone (GDM-D) or diet and insulin (GDM-I). Main outcome measures Maternal (eg.preeclampsia, pregnancy induced hypertension (PIH)) and infant outcomes (eg.mortality, prematurity, macrosomia, large for gestational age (LGA)) were assessed. Results Women in Group 1 were older with a higher booking BMI compared to Groups 2 and 3. Logistic regression analysis adjusted for baseline BMI, insulin use, smoking status, parity, family history, ethnicity and age determined no significant difference in maternal or infant outcomes for women in Group1 compared to those in Group 2. Women with excessive weight gain had higher rates of PIH, macrosomia and LGA. Conclusion In our population, weight gain less than IOM guideline does not appear to be associated with adverse outcomes. However, further validation through a prospective study with a larger obese GDM cohort is required before these findings could be recommended for routine clinical use.