By about which week does sex differentiation begin, making the sex not yet determinable?

• A. Week 8
• B. Week 4
• C. Week 12
• D. At birth

Answer: (A)

Sex differentiation begins as the embryo starts forming its gonads and reproductive structures. The genetic sex is set at fertilization, but the anatomical separation into male or female development starts around week seven to eight. Until that point, the sex cannot be reliably determined by appearance. External genitalia differentiation continues into weeks 9–12, and broader determination is more evident later in gestation. That’s why around the eighth week is the best answer: it marks the onset of differentiation, when the sex is not yet determinable by phenotypic features. Week 4 is too early, week 12 is after differentiation has begun, and at birth the sex is typically observable.