An egg is released from a woman's ovaries roughly once every four weeks. It then travels down a passage called a fallopian tube to reach the womb. If it has been fertilised by a sperm, it may then implant in the lining of the womb and start to grow.
If not, the egg is rejected along with the lining of the womb and a little blood and the woman gets her 'period'.
However, the egg is only in the right place to get fertilised for a few days each month, usually around the middle of the cycle. Some people have a cycle of nearer five weeks, others slightly less than four, and others have cycles that fluctuate between the two.