As Degree Directory explains, there is a certain amount of freedom in the choice of an undergraduate major on the path to becoming an OB/GYN, with anatomy, biology, genetics and various others satisfying the requirements. The next steps are to enroll in medical school and complete an OB/GYN residency.
As Health Communities explains, an OB/GYN is a physician specialized in both obstetrics (pregnancy, labor and after-birth care) and gynecology (health and treatment of the female reproductive system). Typically, the education and training for both fields occurs concurrently. Due to their broad specialties, OB/GYNs are quite flexible and can serve as primary physicians, consultants to other physicians, in private practices or in a teaching capacity.