It's time for your swimming lesson, and you and all your friends arrive at the pool in your bathing suits. As you wait on the edge of the pool for the signal to dive in, you suddenly notice that some of the kids in your class have bellybuttons that stick out a little. Your navel goes in, like a small dimple in the middle of your stomach. Another swimmer has one that's practically flat.

What's up? Is something wrong with your navel -- or is it the other guy who has a problem?

You're all okay. Each kind of navel you see as you wait to go swimming is normal. They formed during the first few weeks after birth. But they actually got started long before that.

As you know, you spent about nine months living inside your mother's body before you were born. Have you ever wondered how you got the food you needed to grow while you were living in her uterus, or womb? After all, you couldn't go out for a hamburger. And have you ever wondered why you could survive inside your mother's body without air to breathe?

When you were a fetus, or unborn baby, you received nourishment and oxygen from your mother's blood. Her body shared the food she ate and the air she breathed with you. That's why your mother was probably very careful about what she ate during her pregnancy. She knew the nutrients she consumed were helping you grow strong and healthy.

To reach your growing body, nutrients and oxygen passed through a twisted tube called the umbilical cord. This cord contains two large blood vessels called arteries, and one smaller vessel called a vein. At one end, the tube was connected to the midsection of your body. At the other end, it was attached to your mother's body.

During your mother's pregnancy, the nutrients you needed to grow passed from your mother's body passed through the vein in the umbilical cord, and reached you. The vital substances were carried to you in her blood. From it, the cells that formed your body soaked up everything they needed to grow.

As cells use nutrients, they produce waste products. These products must be removed from the body. That's the job the two arteries in the umbilical cord do. They carry waste products away from the growing baby.

It might help to understand this process if you think of the umbilical cord as a two-way street. You can imagine that one lane -- the vein -- is jammed with food trucks delivering nourishment to the growing baby. In the other two lanes garbage trucks busily drive along, carrying waste products away from the growing fetus.

The umbilical cord you depended on for food and oxygen developed very early in your mother's pregnancy. As you grew, it was constantly at work. But once you were born, you didn't need the umbilical cord anymore. Your lungs started working, and you started eating on your own. So what happened to it?

In the delivery room, a doctor or a nurse put a clamp on the cord quite close to your stomach. Then the cord was cut. This didn't hurt, because the cord doesn't have any feeling in it. When you came home from the hospital, you still had a piece of the umbilical cord sticking out of your stomach. If you have a younger brother or sister, you may have seen one of these.

After a couple of weeks, the leftover cord dries up and falls off. It leaves a scar behind -- a kind of reminder that you were once attached to your mother. That scar is your bellybutton -- though your doctor might call it by its fancy medical name, the umbilicus.

Different people develop different kinds of bellybuttons after their umbilical cords fall off. That's because of the different ways the cords were attached before birth.

Some fetuses have cords that meet the skin of their abdomens in a neat line. They're the ones who end up with flat bellybuttons.

Some umbilical cords don't quite meet the skin on the fetus's stomach. A short piece of special skin called a membrane attaches the cord to the baby. These babies end up with bellybuttons that collapse into the body a little -- the kind you probably call an inny.

Other fetuses have a little bit of extra skin on their stomach, which grows up around the end of the cord. These babies grow up to have bellybuttons that stick out a little bit -- or outies.

Which kind are you? Tips for Parents

Children have a way of asking questions about the body and human reproduction which you may find difficult to answer. These five books can help. They're recommended by Phyllis Sidorsky, Lower School Librarian at the National Cathedral School for Girls in Washington. This year, Sidorsky is also a member of the American Library Association committee to select the Notable Children's Books of the Year.

*"The Facts of Life," by Jonathan Miller and David Pelham, Viking Press, $18.95.

*"How You Were Born," by Joanna Cole, Morrow Junior Books, $10.25.

*"It's a Baby!" by George Ancona, Dutton, $10.95.

*"Where Do Babies Come From?" by Margaret Sheffield, Knopf, $10.95.

*"The Wonderful Story of How You Were Born," by Sidonie M. Gruenberg, Doubleday, $10.95.