Video narration
Your body has changed as you've grown-but how does a new human begin? The testes make sperm, and the ovaries release eggs. A sperm can meet an egg in an oviduct, a tube leading toward the uterus. When they join, fertilization has occurred. But fertilization does not guarantee pregnancy. And fertilization is only the start. The first cell, called a zygote, begins dividing as it moves toward the uterus. If it attaches there, it can continue developing. It is then an embryo, an early stage of development. The placenta, an organ that develops during pregnancy, passes oxygen and nutrients from the pregnant person's blood to the developing baby, and carries waste away; their blood does not mix directly. Development continues in the uterus until birth. A new human begins when a sperm fertilizes an egg; the zygote then develops in the uterus until birth.