Introduction Women, their Offspring and iMproving lifestyle for Better cardiovascular health of both (WOMB) project is the follow-up of the LIFEstyle study, a randomised controlled trial in obese infertile women, and investigates the effects of a preconception lifestyle intervention on later health of women (WOMB women) and their children (WOMB kids). Methods and analysis Obese infertile women, aged between 18 and 39 years, were recruited in 23 Dutch fertility clinics between June 2009 and June 2012. The 284 women allocated to the intervention group received a 6-month structured lifestyle programme. The 280 women in the control group received infertility care as usual. 4 to 7 years after inclusion in the trial, all women (n=564) and children conceived during the trial (24 months after randomisation) (n=305 singletons and age 3-5 years) will be approached to participate in this follow-up study (starting in 2015). The main focus of outcome will be cardiovascular health, but the dataset comprises a wide range of physical and mental health measures, diet and physical activity measures, child growth and development measures, biological samples and genetic and epigenetic information. The follow-up assessment consists of three stages that take place between 2016 and 2018, and includes (online) questionnaires, accelerometry and physical and behavioural measurements in a mobile research vehicle. A subsample of 100 women and 100 children are planned for cardiac ultrasound measurements. Ethics and dissemination The protocol of this follow-up study is approved by the local medical ethics committee (University Medical Centre Groningen). Study findings of the WOMB project will be widely disseminated to the scientific community, healthcare professionals, policy makers, future parents and general public. Trial registration number The original LIFEstyle study is registered at The Netherlands Trial Registry (number 1530).