Which hormone is the primary determinant of whether a baby will develop male or female external genitalia?

Androgen hormone is the primary determinant of whether a baby will develop male or female external genitalia.

Androgen is usually a steroid hormone. The predominant and most active androgen in testosterone, which is produced by the male testes. Some androgens are naturally produced in the body and if the body doesn't make them properly they can be obtained through prescription medication.