Introduction: The prevalence of childhood overweight and obesity has risen from just 4% in 1975 to more than 18% in 2016. Little is known about adiposity of the infant, but it positively associates to the mother's BMI. Globally, the prevalence of overweight has tripled since 1975 and is now affecting one of three Danish women at the time of pregnancy. However, despite increasing awareness of obesity and accumulating evidence of its health consequences limited effect of intervention in childhood obesity exists. Maternal obesity before conception, an excessive increase in body weight during pregnancy and physical inactivity are some of the risk factors suspected of infant adiposity. Lifestyle interventions during pregnancy have shown limited or no effect in the offspring. Therefore, renewed effort to improve the prevention of childhood obesity is warranted. Methods and analysis: This is a randomized, parallel group, tailored, multifactorial lifestyle intervention trial in women (age 18 to 38 years) with overweight or obesity (BMI 27 to 42 kg/m2) seeking pregnancy. The women are randomized 1:1 to either the lifestyle or standard of care group. The lifestyle intervention is initiated prior to pregnancy. The lifestyle intervention is set off with a low-calorie diet for 8 weeks and throughout the intervention period (prior to and during pregnancy) participants follow an intervention containing exercise according to the World Health Organization guidelines, healthy diet and mentorship to maintain healthy weight before and during pregnancy.The primary outcome is the difference in neonatal adiposity measured at birth. Finally, a child and family cohort will be established to follow the children throughout childhood for healthy weight development. The study will provide evidence of effects from pre-conception-initiated intervention and have the potential to improve health and quality of life for children. Ethics and dissemination: The trial has been approved by the ethical committee of the Capital Region of Denmark (H-22011403) The trial will be conducted in agreement with the Declaration of Helsinki and monitored to follow the guidelines for good clinical practice. Results will be submitted for publication in international peer-reviewed scientific journals.