Maternal diet, physical activity (PA) behaviours, and gestational weight gain (GWG) are important for optimum health of women and their babies. This secondary analysis of the GLOWING pilot cluster trial explored these among women living with obesity in high deprivation. Pregnant women completed food frequency, PA and psychosocial questionnaires. Weights were retrieved from medical records and measured during routine appointments with midwives. Descriptive and regression analyses were stratified by obesity class. A total of 163 women were recruited; 54.0% had class 1 obesity, 25.8% class 2, 20.2% class 3, and 76.1% lived in the two most deprived quintiles. Women had suboptimal dietary intake, particularly for oily fish, fruit and vegetables. PA was predominantly light intensity, from household, care and occupational activities. Most women gained weight outside of Institute of Medicine (IOM) guideline recommendations (87.8%); women in class 3 obesity were most likely to have inadequate GWG below IOM recommendations (58.3%, p 0.01) and reduced odds of excessive GWG compared with class 1 (AOR 0.13, 95% 0.04–0.45). Deprived women with obesity have a double inequality as both increase pregnancy risks. This population requires support to meet guideline recommendations for diet, PA and GWG. Further research exploring obesity classes would inform policies and care to achieve the best pregnancy outcomes.