A timeline of sexual development
Fertilization of Egg and Sperm (Conception)
Female: Zygote forms with XX sex chromosomes.
Male: Zygote forms with XY sex chromosomes.
5 Weeks Post-Conception
XX zygote is anatomically indistinguishable from XY.
Sexual differentiation begins with expression of testosterone in fetal testes. Genital tubercle and gonadal ridge are present. Mullerian and Wolffian ducts are present in both sexes.
Female: Ovaries form from gonadal ridge. Absence of testosterone allows development of Mullerian duct into uterus and vagina. With no testosterone present, Wolffian duct withers.
Male: Testes begin to form from gonadal ridge. Presence of testosterone promotes development of Wolffian duct into prostate. Male hormones shrink Mullerian duct.
Female: Genital tubercle begins to develop as clitoris.
Male: Testosterone causes genital tubercle to develop as penis.
Female: Uterus fully formed; ovaries contain primitive egg cells within 4-5 million follicles.
Male: Testes begin descent from pelvis into scrotum.
Female: Number of follicles within ovaries peaks at 6-7 million and begins to decline.
Male: Testes are completely descended into scrotum.
Female: Between 400,000 and 2 million primary follicles are intact.
Puberty (10-14 years old)
Female: Number of primary follicles has dwindled to 400,000. Onset of menses and ovulation—primary follicles begin to mature monthly into ova. Release of estradiol spurs breast development.
Male: Testes produce 100 million sperm daily for the rest of life. Testes and penis grow larger. Prostate grows rapidly, then slows after puberty.