Objective: Women with severe mental illnesses are a vulnerable population and little is known about their reproductive planning needs. The aim of our study was to describe rates of unintended pregnancies, postpartum contraception, identify use and knowledge of prenatal/pregnancy vitamins and identify modifiable lifestyle risks. Design: Mixed methods study incorporating a cross-sectional survey and prospective pregnancy data collection Setting: A multidisciplinary antenatal clinic in Australia Method: Thirty-eight pregnant women with severe mental illnesses: schizophrenia, schizoaffective, bipolar and severe post-traumatic stress disorder Main outcome measures: Unintended pregnancy rates, immediate postpartum contraception, use of prenatal and pregnancy vitamins and knowledge sources, obesity, and use and cessation rates for smoking, and substances, and comorbid medical conditions Results: Overall 42% of women had unintended pregnancy, with those with schizophrenia at most risk (56%). A long acting reversible contraception was inserted in 5 women (13%), with 45% having no immediate contraception prescribed prior to postnatal discharge. Women’s main source of vitamin supplementation for pregnancy was from general practitioners. Prenatal folic acid use occurred in 37%, with rates differing for those with a diagnosis of bipolar disorder (52%) and schizophrenia (25%). Vitamin deficiencies occurred in pregnancy, with iron deficiency (ferritin <30 ng/mL) (n = 27, 73%) the most frequent. Overall 21% of women smoked cigarettes and 35% were obese. Discussion: Addressing gaps in use of effective contraception, proactive reproductive planning and lifestyle management may improve outcomes for women with mental illnesses and their babies.Key points Women with severe mental illnesses have complex health needs that require targeted reproductive counselling. This study adds to what is known by highlighting that: •Women with schizophrenia appear more likely to have unintended pregnancy. •Prenatal counselling for women with severe mental disorders should include recognition and optimisation of management for the high rates of pre-existing medical comorbidities, obesity and elevated nicotine and substance use. •Many women with severe mental illness need increased doses (5 mg) of prenatal folic acid due to psychotropic medication risk and obesity, as well as treatment for high rates of iron and vitamin D deficiency in pregnancy.