Elizabeth knew that human babies are born after nine months of waiting. She had learned that in her fourth-grade science class. So when her mother had to be rushed to the hospital during the eighth month of her pregnancy, Elizabeth was worried.

At her grandmother's house, Elizabeth lay awake wondering what was going to happen. When her grandmother came in to check on her, she said, "Will the baby be all right?"

"I hope so," said her grandmother. "I think it will be, because premature babies get very good medical care now. When I was your age, premature babies didn't have as much of a chance."

Premature babies -- sometimes nicknamed "preemies" -- are babies born too early. In the United States, about 7 percent of all babies are premature. When they come into the world they aren't as well prepared for life as full term babies, who grow in their mothers' bodies for a full nine months. Preemies often have underdeveloped lungs, and they may not weigh very much. These tiny babies need very special care during the first few weeks of their lives.

As Elizabeth and her grandmother were talking, the phone rang. It was her father calling from the hospital. "You have a sister," he told her. "Her name is Sarah, and she's very tiny. The doctors say that she's going to be okay, but she'll have to be in the hospital for a while."

Elizabeth was happy. She had hoped for a sister -- although a brother would have been okay, too. As she lay in bed trying to fall asleep, she thought about the things she and Sarah would do in the future.

The next day, Sarah went to the hospital to see her sister and her mom. Her mom looked great. But Sarah, who was lying in a special covered bed called an incubator, looked awfully skinny. Her little arms and legs were very thin, but she waved them around vigorously. There were tubes and wires hooked up to her body. Elizabeth asked why the tubes and wires were there.

"Those help Sarah breathe, and keep track of her temperature and heartbeat," a nurse explained. "We want to keep track of how she's doing."

"She's so tiny I'd be scared to hold her," Sarah said.

"You'll get to hold her when she has gained some weight," the nurse said. "She'll look stronger then."

Elizabeth's mom came home from the hospital without Sarah. She seemed pretty sad at first. When she looked into the empty nursery with its half- wallpapered walls, she started to cry.

"I'm a little worried about Sarah, and I miss her," Elizabeth's mom said. "I keep thinking I must have done something wrong when I was pregnant that made her come too early."

But the doctor had explained to the family that no one did anything wrong. Some babies just arrive too early even when the mother does everything right during a pregnancy. Elizabeth's mother had been very careful to get good medical care, eat well, get lots of rest, and avoid smoking cigarettes and drinking alcohol. But Sarah showed up early, anyway.

Over the next two weeks, Sarah improved. She gained weight, and got strong enough to breathe without help from a tube. Sarah's mom and dad spent many, many hours at the hospital visiting their "preemie." Elizabeth sometimes felt a little jealous about all the time Sarah was taking up.

At home, Elizabeth and her mom finished the nursery. They hung pictures, and decided where to put the crib. They washed baby clothes and folded them up in drawers. "Sarah could be home in a week," said Elizabeth's mom.

When Sarah came home, she still looked very small. Even Elizabeth's mom and dad were a little nervous about taking care of her. "I guess we got used to letting the nurses take care of her," Sarah's mom said.

To get used to caring for their fragile daughter, and to share their feelings about having a preemie, Elizabeth's parents joined a group of other "Preemie Parents." It made them feel better to know that other people had special babies, too. And they heard stories about how other preemies had grown up to be strong, healthy children. Those stories gave them hope.

At first, Elizabeth was afraid to touch her sister. She was scared that she might hurt her. Sarah would stand by Sarah's bed and just watch her breathe as she slept. Then one day Elizabeth reached out and stroked her sister's little arm. She put her finger down near the baby's hand.

"Mom, Sarah's holding my hand," Elizabeth said. The baby had gripped her sister's finger hard. "I can feel her fingernails!"

"I think she likes you," Elizabeth's mother said. Tips for Parents

The National Committee for Prevention of Child Abuse (NCPCA) publishes a variety of booklets about parenting and families, including a booklet about prematurity written for kids, "My Brother Got Here Early." Write to NCPCA, Publishing Department, 332 S. Michigan Ave., Suite 1250, Chicago, Ill. 60604-4357, (312) 663-3520.

Catherine O'Neill is a contributing editor to the Health section.