Background: To assess the type of infant nutrition at initiation of first feed in association with increasing maternal pre-pregnancy Body Mass Index in an Australian obstetric population. Methods: A retrospective cohort study from 2008 to 2013 was undertaken. Body Mass Index was available for 12,347 women categorised into groups according to: underweight (≤18 kg/m2); normal weight (19−24 kg/m2); overweight (25−29 kg/m2); obese class I (30−34 kg/m2); obese class II (35−39/kg2) and obese class III (40+ kg/m2). Type and initiation of infant feeding is routinely recorded in the study hospital's birthing outcomes system. Six body mass index categories and mode of infant feeding were examined using logistic regression. Confounding factors that were controlled for included smoking status, parity, country of birth and maternal age. Results: Within this cohort, 609 (4.93%) women were underweight, 6235 (50.50%) had a normal BMI, 3116 (25.24%) were overweight, 1314 (10.64%) were obese class I, 596 (4.83%) were obese class II and 477 (3.86%) were obese class III. In adjusted models’, as BMI rose, women were significantly less likely to initiate exclusive breastfeeding and more likely to exclusively formula feed. Women with a BMI of 40+ kg/m2 had an AOR of 2.91(CI 1.94−4.25) for initiating exclusive formula at the time of their infant's first feed. Conclusions: Women who are overweight or obese are significantly less likely to initiate exclusive breastfeeding and more likely to give exclusive formula at the time of their newborn infants first feed. Effective breastfeeding interventions are required for these high-risk groups in the days leading up to and within the first hours after birth.