Unlike men, who make sperm continually once they enter puberty, women are born with all the eggs they will ever have.
During each menstrual cycle, one egg matures to allow it to be fertilized if it comes into contact with sperm.
Women are most fertile during certain parts of their cycles. A women's "fertile time" is about one-third of her cycle. During the other two-thirds of her cycle, she is probably infertile.