Around week 8, what developmental process begins?

• A. Gonads develop into testes or ovaries
• B. External genitalia form
• C. Placenta finishes forming
• D. Limb digits form

Answer: (A)

Gonadal differentiation marks the start of sexual differentiation around week 8. At this point the undifferentiated gonadal ridges begin to become either testes or ovaries, driven by genetic signals. The presence of the SRY gene on the Y chromosome pushes the pathway toward testis development (with supporting cells like Sertoli and Leydig cells and the production of male hormones), while its absence leads toward ovary development. This timing distinguishes it from other developments that occur a bit later or earlier: external genitalia form later in the first trimester, the placenta is mostly established by the end of the first trimester, and limb digits begin to form and separate earlier in the first trimester.