A baby girl is born with all the eggs she will ever have; her body does not make any more. Starting at about age 11 or 12, generally one egg will be released each month (called ovulation) to travel through the Fallopian tube and into the uterus. (Under rare conditions, two or more eggs might be released at once. This can cause fraternal twins.) If the egg meets up with a sperm cell, the egg will be fertilized and she will become pregnant.
After about age 45, the woman goes through menopause, after which her body no longer releases an egg each month, and she can no longer become pregnant.
Boys start creating millions of sperm per month in the testicles starting about age 12. Men never stop producing sperm.
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