# Human Reproduction How an egg and sperm meet in the body, and how a fertilized egg travels to the uterus and settles in its lining to begin a pregnancy\. By John Doe Topic: Human Body & Medicine [Open the interactive Jig](https://ask.jig.com/j/DD1CNOGx7sDK) ## Transcript ### 1. Where a new life begins A baby starts when two tiny cells meet: a sperm cell from a man and an egg cell from a woman\. Sex is the most common way they come together\. This model shows the inside of a woman's body, where that meeting happens\. Drag to turn it\. The model is cut open from the front and greatly enlarged\. ### 2. The organs inside The uterus, or womb, is a hollow muscle about the size of a pear, where a baby can grow\. Two fallopian tubes reach out towards the ovaries, which store the eggs\. Below, the cervix opens into the vagina, the passage to the outside of the body\. Each part glows in turn: uterus, tubes, ovaries, then cervix and vagina\. ### 3. An egg is released About once a month, one ovary releases a single egg\. This is called ovulation\. Soft, finger\-like fringes at the end of the tube sweep the egg inside, and it drifts along the tube\. An egg lives only about 12 to 24 hours\. It is one of the largest cells in the body, about the size of a full stop\. ### 4. What happens during sex Sex is something adults choose together, and both people must agree to it\. This agreement is called consent\. During sex between a man and a woman, the penis is inside the vagina\. At the peak, the man releases semen, a fluid carrying tens of millions of sperm, close to the cervix\. Condoms help prevent pregnancy and protect against infections that can pass between partners during sex\. ### 5. A long swim Each sperm swims by whipping its tail\. They travel up through the cervix and the uterus and into the tubes, but most tire or take a wrong turn\. Only a few hundred ever come near the egg\. Sperm can live inside the body for up to five days, so sex in the days before ovulation can also lead to pregnancy\. ### 6. Fertilization Several sperm reach the egg, but only one gets inside\. At once, the egg's outer layer changes so that no other sperm can enter\. Together the egg and sperm carry 46 chromosomes, the instructions for building a whole new person\. 23 chromosomes come from each parent\. ### 7. Dividing on the way The fertilized egg splits into 2 cells, then 4, then 8, without getting any bigger\. The tube slowly moves it towards the uterus, a trip of three to four days\. By then it is a hollow ball of about 100 cells called a blastocyst\. ### 8. Settling in About a week after fertilization, the blastocyst sinks into the thick, soft lining of the uterus\. This is called implantation, and it is when a pregnancy begins\. Its cells start making a hormone that pregnancy tests detect\. If no egg is fertilized, the lining is shed about two weeks after ovulation\. This is a period\. ### 9. Not every time Sex does not always lead to a baby: an egg can only be fertilized during a few days in each monthly cycle\. People use contraception, such as condoms or the pill, when they do not want a pregnancy\. Anyone with questions can ask a doctor or nurse\. The fertile window is about six days: the five days before ovulation and the day itself\.