I’m pretty sure they teach this in school, but in case you need a revision:
When a man and a woman have sexual intercourse (I’m assuming you understand that bit) the man’s semen, which contains sperm cells enters the woman’s vagina. The sperm travel up through the vagina and cervix into the uterus. If they have sex during the fertile part of the woman’s menstrual cycle, an egg may be present and may be fertilised by one of the sperm. The sperm and egg divide into more and more cells, which grow into a baby over about 9 months.
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