Maternal mood disorders are serious complications of pregnancy experienced by as many as 1 in 5 mothers worldwide. Maternal obesity increases the risk of postpartum mood disorders, but the mechanisms are unknown. Here we examined the effects of maternal obesity, induced by the consumption of a high-fat-high-sugar (HFSD) diet before and during pregnancy on postpartum brain and behaviour in rats. We also assessed if the effects of HFSD could be reversed by consumption of a healthier diet during pregnancy, specifically by a diet replete in omega-3 polyunsaturated fatty acids. Our data show that consumption of HFSD before and during pregnancy activated magnocellular, but not parvocellular, neurons in the paraventricular region of the hypothalamus (the apex of the stress axis), and only moderately affected anxiety-like behaviours. However, HFSD-induced pre-conception obesity was associated with elevated levels of inflammatory cytokines and reduced microglial complexity; morphology indicative of microglial activation. A shift to a healthier diet during pregnancy reversed systemic and neuro-inflammation. Surprisingly, both HFSD and omega-3-replete diet increased the numbers of newborn neurons in the hippocampus. While outside of pregnancy neurogenesis refines hippocampal activity, the opposite occurs postpartum, where increased neurogenesis may facilitate mood disorders. These data highlight the potential role of inflammation in mediating the effects of diet on the maternal brain and support the importance of a balanced dietary intake before and during pregnancy. Our findings also indicate the need for future research into key triggers that may influence the neuroimmune balance in the maternal brain.