You only need one sperm and one egg to make a baby, but how do they get together? We find out a little about having sex and then the race is on for a sperm to find and fertilise an egg! While a fertilised egg can make a baby, sometimes it can make two babies, or even more. Some parents can’t make babies by having sex so they need help from a doctor who can use IVF, sperm or egg donation to get that sperm and egg together so it can grow – just like other babies.