Male: By the tenth week of pregnancy, a fetus's gender is evident, and by week 14 sex organs have formed.
Female: At 20 weeks, a female fetus has a fully developed reproductive system, replete with six to seven million eggs.
Male: Boys do not produce sperm until puberty, so a male infant's reproductive system is largely unchanged between the fetal stage and infancy.
Female: At birth, a girl's pair of ovaries contain up to 1,000,000 follicles, which are hollow balls of cells, each with an immature egg (or ovum) in the center. A girl is born with all the eggs she will ever have.
Male: While women are only fertile during a few days in their menstrual cycles, men are constantly fertile from the onset of puberty (usually at 10 years of age) when a surge in testosterone triggers the production of an average of 200 million sperm each day. Sperm cells mature in the epididymis, a 20-foot-long series of thin, tightly coiled tubes. Sperm take from two to 12 days to pass through the epididymis and to develop the ability to swim and fertilize. From start to finish, the production of a single sperm can take up to 72 days.
Female: During childhood, a girl's ovaries absorb almost half of the 1,000,000 immature eggs with which she was born. Of the 400,000 eggs present during her first menstrual period, only 300 to 500 of them will develop into mature eggs across her reproductive life span. Her body reabsorbs the rest before they complete development.
Ovulation and menstruation start near the end of puberty, generally when a girl is about twelve-and-a-half, though any age from nine to 18 is normal. Each month during a woman's reproductive years, ten to 20 follicles begin maturing under the influence of hormones. These hormones, which are low during childhood and increase exponentially during the reproductive years, regulate the entire reproductive process.
Two distinct cycles govern menstruation and ovulation, the uterine cycle (commonly called the menstrual cycle) and the ovarian cycle, which is responsible for ovulation. Hormones coordinate these two cycles, which last from 20 to 40 days each to prepare the lining of the uterus (endometrium) each month for a possible conception.
Ovulation generally occurs about two weeks after the last day of a woman's menstrual period. A follicle discharges a single egg to the surface of the ovary, whence it is slowly carried down the fallopian tube and into the uterus. This process lasts about two days, after which conception is not possible. If the egg reaches the uterus without fertilization by a sperm, it will pass unnoticed out of the woman's body. If fertilization occurs, the egg will embed itself in the endometrium and begin its growth.
Male: Both men and women are normally at their peak of fertility in their early to mid-20s.
Female: Though girls menstruate throughout their teens, many women do not regularly ovulate until their mid-20s, when hormone levels become more regular and women begin to ovulate each month.
Male: Male fertility in the 30s is much the same as in the 20s.
Female: Though many women conceive and give birth to healthy babies into their 40s, conception becomes more difficult for women in their mid-to-late 30s and early 40s, and women over 35 are more likely to carry a baby with abnormalities.
Male: While women's fertility in middle age begins to decrease, men of the same age are still highly fertile.
Female: Menstruation and ovulation continue until the age of 50, on average, but any time between 40 and the late 50s is a normal age to begin menopause. The body changes that occur between the reproductive and post-reproductive phases of a woman's life, most notably the end of monthly bleeding, are called the "climacteric" and often take place over as many as 15 years. During the gradual process of menopause, the ovaries stop maturing eggs and releasing large amounts of hormones. A woman is not considered to have completed menopause until she has passed one full year with no period. Once menopause is complete, a woman can no longer conceive a child.
Male: Men are capable of fathering healthy children throughout their lives. It is not uncommon, however, for older men to experience some sexual dysfunction. In addition, some studies have shown that children conceived by a father over the age of 50 have an increased risk for mental illness, particularly schizophrenia.
Female: Post-menopausal women carry some eggs in their ovaries, but they are not viable and cannot be fertilized. Many women take hormones during and after menopause in order to ease the process of menopause and help prevent post-menopausal bone-density loss. However, hormone-replacement therapy does not maintain a woman's fertility into old age.