Hormones
As puberty hits, the production of certain hormones increases. For women, this begins in the ovaries, located on either side of the uterus. During puberty, the ovaries begin to produce more estrogen. The estrogen kick-starts the menstrual cycle (including the period). For men, puberty causes the testicles to increase their production of testosterone. This added testosterone sparks the production (also in the testicles) of more sperm. Millions of sperm are stored in the testicles.
Setting the Stage
The ovaries contain about a million eggs--which have been there since birth--and the menstrual cycle involves the release of one of these eggs (or ovums) each month. The egg travels along the Fallopian tubes (which connect the ovaries to the uterus) for three to four days until it reaches the uterus. As this is happening, the endometrium, or lining of the uterus, becomes thick with fluid and blood (to prepare a suitable home for a growing baby). If a female has sexual intercourse with a male during this time, pregnancy is very possible.
Fertilization
At the peak of sexual intercourse, the male ejaculates, releasing millions of sperm through the woman's vagina toward the uterus. Some of the sperm reach the uterus, where one meets the egg, fertilizing (or joining) it. This fertilized egg then attaches to the wall of the uterus. (If the egg is not fertilized, the egg as well as the thick lining that has recently developed will shed themselves through the vagina.)
Development
The human fetus requires about nine months within the womb, or uterus, to prepare itself for birth. During this time, the baby is nourished by whatever the mother brings into her body, reaching the fetus via a tube called the umbilical cord. When the baby is ready for birth, the muscles of the uterus begin contracting (tightening and loosening) in preparation for the actual pushing out of the baby. During labor, the muscles of the vagina, formerly a narrow tube, widen enough to allow the passage of the baby and, with the delivery of the child, the reproductive system has completed the process of reproduction.
