Various hormones, chemicals, and teratogenic agents exhibit gender-related effects in utero as well as postnatally. Among such gender-specific teratogens are endocrine disruptors, especially phthalates that affect male gonads, diabetes-induced oxidative stress with more deleterious effects on male offspring, procarbazine-induced cleft palate affecting more male fetal rats compared to females, and VPA-induced autism-like behavior that affects differently males than females. Hence, there are many needs for the accurate determination of genetic gender. In newborn animals, the morphological methods that exist for sex determination i. Hence, an accurate and simple method for the prenatal and early postnatal assessment of the genetic sex, prior to reliable evaluation from the external genitalia, is of utmost importance. Indeed, several methods have been developed for accurate assessment of genetic sex, which are discussed in this chapter.