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The development of a functional reproductive system is one of the most critical features of an organism because it is directly related to its genetic fitness. An individual's genetic legacy is passed onto subsequent generations via the germ cells housed in the developing gonads.[1] Sexual development involves two distinct developmental processes: sex determination and sex differentiation. Sex determination is the developmental assignment that directs the undifferentiated zygote to progress into a sexually dimorphic individual (towards male or female).[2] In humans, chromosomal sex is determined at fertilization when a sperm contributes either an X or Y chromosome to the X chromosome in the oocyte. Sexual differentiation is the developmental process and pathway towards developing male or female phenotypes from undifferentiated embryonic structures. Sex differentiation typically develops along a pathway consistent with the chromosomal sex of the embryo. Sex differentiation involves multiple levels: chromosomal, gonadal, hormonal, phenotypic, and psychological differentiation. At the genetic level, chromosomal sex is determined by the chromosomal complement after fertilization, where XY indicates a chromosomal male and XX indicates a chromosomal female. Until approximately the sixth week post-fertilization, no sexual difference is observable in a chromosomally male or female conceptus. The bipotential gonads are the first to differentiate and are morphologically indistinguishable early in development. Gonadal differentiation into either ovaries or testes is an important part of sex development, as a functioning gonad and the hormones they produce impact the development and differentiation of an individual's internal genitalia, external genitalia, and secondary sex characteristics.[3][4]