The human reproductive system develops partially before birth, stays the same throughout early childhood, and develops further during preadolescence and adolescence. Puberty in boys and girls is divided into five distinct stages called Tanner Stages.
Boys Before Birth
The genitals start out looking the same in male and female fetuses until males start developing male genitals between theseventh and 13th weeks of pregnancy. By the 15th week of pregnancy, the size of the genitals in males and females is about the same, so the differences cannot be seen on ultrasound until later in the pregnancy. In male babies, the testes develop near the kidneys and start to descend through the inguinal canals around the last half of the seventh month of pregnancy, and reach the scrotum before birth.
Girls Before Birth
While males do not begin to produce sperm until puberty, females begin making egg cells (oocytes) before birth. At birth, a baby girl has all of the egg cells she will ever have: between 200,000 and 2,000,000 primary oocytes in each ovary. She will still have about 400,000 primary oocytes at puberty, and about 400 of those will develop into mature egg cells for ovulation.
Puberty for Girls
According to the Medem Medical Library, the first Tanner Stage of sexual development in girls starts at age 8 to 12, when there are no visible outer signs of changes. During this stage, the ovaries grow larger and begin releasing hormones.
The second Tanner Stage for girls starts sometime between age 8 and 14, when girls begin a "growth spurt" and get taller and heavier very quickly. Girls develop small breast buds that may be tender, and the sweat and oil glands in their skin become more active, which may lead to acne. Girls also start to grow fine hair in their underarms and genital area.
The third Tanner Stage in girls begins between ages 9 and 15. A girl's hips may start to become wider than her waist, and her breasts get fuller and rounder. She continues to grow taller and heavier, and starts to have clear or white secretions from the vagina. Her pubic hair gets thicker, darker and curlier, and she may have her first period (menarche).
Stage 4 in girls starts between ages 10 and 16. During this stage a girl's underarm hair gets darker, and her pubic hair forms a triangular patch. The nipples and areolae may start to stick out. Many girls have their first period (menarche) during this stage.
Stage 5 begins age 12 to 19. This is when breast and pubic hair development is complete, and when most girls reach their adult height. Most girls now have regular periods.
Puberty for Boys
According to the Medem Medical Library, the first Tanner Stage of sexual development in boys occurs between ages 9 and 14. Boys begin to produce hormones during this stage, but there are no outward visible signs of changes.
Tanner Stage 2 in boys starts between age 11 and 13. In this stage, the scrotum turns a darker color and hangs lower, and the testicles become larger. Boys have a "growth spurt" and grow taller and heavier. They may start to grow hair in the underarms and on the legs, and fine hair begins to grow at the base of the penis.
Tanner stage 3 in boys starts between age 12 and 14. Boys grow larger muscles and broader shoulders, and the genitals grow larger. Their pubic hair gets darker, thicker and curlier, and they may begin to produce sperm. They continue to grow taller and heavier, and grow more hair in their underarms and on their legs. They may have some temporary swelling and tenderness around the nipples.
In Tanner Stage 4, which begins between ages 13 to 16, boys continue to grow taller and heavier. Their voices crack, then become deeper. Their pubic hair becomes coarser and darker, and their genitals continue to grow. Most boys have started producing sperm.
Tanner Stage 5 in boys starts between age 14 and 18. Boys reach their adult height, start developing facial hair, and may develop chest hair. Their genitals have reached adult size, and their underarm, pubic and leg hair are the color, thickness and texture of a mature adult's.
Considerations
Every child develops differently. Most children who are called "late bloomers" develop into perfectly normal adults, which is why each of the Tanner Stages has such a large age range. See Resources for answers to frequent questions that children and their parents have about puberty and diagrams of the Tanner Stages.
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