Development of fetal genitals: click across this timeline to view how the genitals develop in the womb for female and male fetuses.
At 6 weeks of gestation, all fetuses have the same "protogenitals," which resemble neither female nor male genitals. These include the gonads, tubercle, and swelling.
At 12 weeks, the protogenitals have developed about halfway to their fully developed form. In this case, the swelling has largely remained in place, with a hole forming between them. What sex do you think this fetus is?
At 20 weeks, the fetus has developed fully functioning genitals. You can now see this is a female fetus, with a clitoris, labia, and ovaries.
A comparison of female and male genital development: the tubercle becomes either the clitoris or glans penis, the swelling becomes either the labia or scrotum, and the gonads become ovaries or testicles.