While physical, human-like characteristics are not immediately visible in a fetus, many features are already set within hours of fertilization. This includes personality traits, eye color, hair and gender. External features like ears, fingers, toes and eyes begin to take shape within a month – before most women know they are even pregnant. Internally, the foundation of the child’s bones and ligaments has been laid. By 20 weeks gestation, female fetuses completely develop their reproductive system, replete with every egg they will carry throughout their lifetime.