This process starts before the developing child is even old enough to be considered a fetus, and is instead still an embryo. A developing human is not considered a fetus until the 9th week of development in the uterus, whereas sex differentiation begins during the 6th week of pregnancy. By the sixth week of development, all embryos have both Wolffian ducts and Müllerian ducts. The Wolffian ducts are embryonic structures that can form the male internal reproductive system.