What is makes a fertility doctor, or an infertility specialist, different from an ob/gyn?

Fertility doctors, called reproductive endocrinologists, treat infertility as well as other reproductive issues such as hormonal disorders, menstrual irregularities, or recurrent miscarriage. In order to be considered a fertility doctor, he or she must complete a two-to-three year fellowship in reproductive endocrinology, and must pass oral and written exams.