Additional years. Mfm is a sub-specialty of ob/gyn. Ob/gyn takes 4 years of training after medical school. Mfm (maternal-fetal medicine) takes an additional 3 years of training after that in which the focus is on managing different types and situations of high-risk pregnancies. An mfm doctor is an ob/gyn but with additional specialized skills not seen in standard ob/gyn practice.