MARCH aims to understand the impacts of maternal characteristics and behaviours during pregnancy on maternal and child health outcomes. Participants include over 1,000 women who were recruited from 2017 to 2023 during their first prenatal appointment in Michigan, United States of America. In addition, over 900 of their children have also been included in the study. A baseline prenatal interview was conducted immediately after enrolment, and follow-up occurred during the second and third trimesters. At childbirth, data were collected from both mother and child. Post-natal data are gathered via phone and online surveys conducted when the child is three months, nine months, two years, and annually thereafter.