Wonder Years

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.

6-7 Weeks

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.

7 Weeks

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.

8-12 Weeks

Female: Genital tubercle begins to develop as clitoris.
Male: Testosterone causes genital tubercle to develop as penis.

3-5 Months

Female: Uterus fully formed; ovaries contain primitive egg cells within 4-5 million follicles.
Male: Testes begin descent from pelvis into scrotum.

6-7 Months
Female: Number of follicles within ovaries peaks at 6-7 million and begins to decline.
Male: Testes are completely descended into scrotum.

Birth

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.