In a small, cluttered office in a Prince George's County health clinic, nine pregnant teen-agers are crammed together for their weekly class on prenatal care. They ignore the crying of a baby in a nearby waiting room as they stare intently at a film being projected on the white cinder block wall.

Cassandra Bratsis, 19, rubs her swollen belly as she listens to four school-aged mothers in the film tell their stories.

"When you have nothing, a baby is something," said one.

"Before my baby was born I wouldn't think of giving her to anyone. But now . . . " says another young mother who had to drop out of school and look for work.

Afterward, the teen-agers decide the film was helpful but "pessimistic." Most said they are looking forward to the births of their own children.

Susan Nalls, the instructor and mother of a 17-month-old boy, understands. "Having a baby is a very special event even if your situation isn't," she said.

In 1979, of the 2,037 Prince George's teen-agers who became pregnant, 1,370 gave birth and 651 had abortions, according to state health statistics. In 16 cases, the babies died before birth. Statistics for 1980 and 1981 are still being compiled.

A new county program aimed at helping these teen-aged mothers-to-be provides free prenatal instruction. It is called the "Early Bird Program" -- as in "the earlier one gets prenatal care the better are one's chances of having a healthy baby," according to Vanessa Herrington Winborne, director of the project.

The series of six classes is repeated every eight weeks at six locations throughout the county. Topics include nutrition, body changes, fetal development, preparing for childbirth, infant care and decision-making.

The Early Bird project, administered through the county health department, is financed by a $13,333 grant from the March of Dimes, an organization founded to help prevent birth defects. The grant, which began in March and runs out in August 1982, may be renewed.

Nalls' class is as much sex education for some of the teen-agers as it is prenatal instruction. Before the classes begin, Nalls said, many of her students don't know the difference between their cervixes and their vaginas. By the time the classes are finished, students have learned about their own anatomies as well as how their babies are growing inside of them and how to prepare for delivery.

The classes also serve as a means to dispel many myths about pregnancy and birth control. In one class, for instance, Nalls encouraged a student to buy a crib. The student said she didn't want to because she was afraid of crib death.

Most misconceptions, however, surround birth control, said Nalls, citing girls' reliance on such tactics as "coke douches" and standing up immediately after intercourse.

The Prince George's County schools offer sex education classes but the subject is not required for graduation. The classes are offered as electives and students who enroll must have written parental permission. Last year, 9 percent of the county's high school students took sex education, said Dr. Floss Fenton, supervisor of health education and services for the schools. She supports the Early Bird classes, Fenton said, and staff members at the schools tell pregnant students about the program.

In the classes, Nalls conveys an enthusiasm for motherhood. She encourages her students to ask questions and provides an atmosphere free of judgment on the circumstances surrounding the teen-agers' pregnancies.

Cassandra Bratsis, whose baby is due next month, said she practices the labor-easing breathing techniques she learned in class, and that she has learned many other things in the sessions. She said she and her boyfriend read the pamphlets that Nalls hands out.

At a recent class, Gwen Robertson, 19, pregnant with her second child, talked about the difficulties of being a mother at 15. "Me and my daughter were like sisters. She used to call me Gwen and my mother, mother," she said.

Another student said she wanted her boyfriend with her in the delivery room but was afraid a fight might ensue because her mother was angling to be the one "helper" allowed by the hospital.

The first series of Early Bird classes ended in September. Director Winborne said she was pleased that 36 teen-agers took part in the program. The enrollment doubled for the second series of classes.

Bratsis completed the first series and is now attending the second group as a "peer helper," the teen-aged mother who assists Nalls with the class.

Traditionally, said Carmine Valente, director of the county health education department, programs similar to the Early Bird project have had a dropout rate of 40 to 50 percent. With the peer helpers, who go through six hours of training, Valente's office is trying to lower the rate to 30 percent. Peer helpers receive a $50 stipend.

At the end of the six weeks of classes Nalls conducts two community workshops on parenting and other issues.

The teen-agers are encouraged to bring a friend or relative to any of the sessions.

Francisco Roman, 33, Bratsis' boyfriend, attended a recent session. He said he wished there were similar programs for fathers-to-be.

"There's a lot of things I'm going through that I never felt before," said Roman, who teaches sign language through the Montgomery County recreation department. "I need someone to talk to. Men need some kind of rap session to air their feelings. Maybe they would be a lot more supportive if they didn't feel so isolated, so out of the picture."

He said the months that Bratsis has carried the baby have given him time to get used to the idea of being a father..

More information on the Early Bird classes may be obtained by calling the county health education office at 386-0271.