Fellowship in maternal-fetal medicine, can i get a fellowship in gynecologic oncology afterward?

Sure: You can do as much training as you want. You just have to be accepted into a program for it.

Yes: These are 3-year fellowships a piece so after 4 yrs of medical school, 4 yrs of residency in ob/gyn, 3 yrs of a maternal fetal medicine, if you have the time, you can apply and complete another 3-year fellowship. At some point in time however, you will need to make a living to pay off loans if you have any, and it would be difficult to maintain your certifications in two different fields.

Theoretically yes...: In practice, few if any physicians can endure 11 years of training and resident pay. Additionally, most program directors would frown upon the switch in direction as a sign of personality imbalance and/or unpredictability and would be reluctant to hire you after the first fellowship without a very good reason. Given that both subspecialties are now quite competitive, i advise against trying this.

Fellowships: You can do a fellowship in a sub specialty after completing a 4 year residency in OB/GYN. A fellowship in Maternal-Fetal Medicine takes 3 years. A fellowship in Gynecological Oncology takes 3 or 4 years depending on the program. You can do a second fellowship after completing another fellowship.