Sean Stewart and Antoine Montgomery, 16-year-old juniors at Petersburg (Va.) High School, can't say enough about teenage pregnancy in general and "the belly" in particular.

"At first, all I was really thinking about was sex," says Stewart. That was before his encounter with the belly. "What a big change. It ties you up. It'd be hard just getting in and out of a car. If you think about sex with a girl, you have to think about making a girl pregnant."

Says Montgomery: "Yeah. It was heavy. I don't want to get a girl pregnant. Not until I'm old enough. If any guy wore the belly he'd understand why you have to change."

Stewart and Montgomery are participants in the Adolescent Pregnancy Prevention Program in Petersburg. The full name of the belly they are so impressed with is "Empathy Belly Pregnancy Simulator."

Some people who encounter it love it. Some think it's a fad; others claim it is potentially harmful. Its designer, Linda Ware, says she developed it to educate and enlighten expectant fathers: "The men just weren't getting it," says Ware, a childbirth educator, counselor and birth assistant for 15 years as well as founder of Birthways Childbirth Resource Center Inc. in Redmond, Wash. "That's why I started working on my first prototype. One of my clients was a pilot and invited me to the base to see his F-15 flight simulator. I thought, 'Simulation is a great way to teach people.' I thought it would be great if we could simulate the pregnant condition."

Women love the belly, Ware says, because it promotes an "appreciation and understanding for their condition."

Deborah Davis, director of the adolescent project in Petersburg, by the way, uses the belly in her all-male program but not in the all-female group, explaining that she fears the girls would put it on and get excited rather than concerned. "They have a more romantic view," says Davis. "They think, 'My face will clear up; my hair will be shinier.' Because boys don't actually go through this, they don't concentrate on the image."

The device is modeled after the belly of a woman in her eighth month of pregnancy. The vest, complete with "breasts," weighs around 35 to 40 pounds. It is strapped onto the torso and includes a lung constrictor (rib belt) the pregnant belly (a water-filled vinyl bladder) and strategically placed pockets that contain weights.

The weights -- from 1 to 6 pounds -- are meant to feel like the fetal limbs and head. One pocket contains a 2-pound suspended weight designed to simulate fetal movement. The rib belt puts pressure on the bladder, resulting in an increased sense of urgency and frequency of urination.

A person wearing the belly will experience 20 of the typical symptoms and effects of pregnancy.

While instruction techniques may vary, some teachers like to bring in a load of laundry, dump it on the floor and casually ask the wearer to bend over and pick it up. There's also usually a shoe-tying exercise, and if a mat or couch is available the volunteer is asked to lie down and quickly get up several times.

The belly costs around $600. Optional, extra-cost instructional videos -- one designed for the fathers-to-be and one for teenagers -- also are available.

So who's buying? Hospitals are purchasing it as an aid in their birthing classes but, according to Birthways market analysis, it is high schools and pregnancy prevention programs that are clamoring for the belly. Says Ware, "Our strongest buyers are high schools and Planned Parenthood."

Birthways has sold 300 units since the belly hit the market in 1989, and now averages 35 to 40 per month. It currently is being shipped to Japan, Germany, Puerto Rico, Venezuela, Canada and Australia.

Marilyn Faris Scholl, home economics supervisor with the Arlington Public Schools, ordered a belly for her school system last spring and introduced it to her home economics teachers the first week of school this month. "We emphasize interpersonal relationships, child development and parenting," notes Scholl. I read about the Empathy Belly last year and thought it was a good piece of equipment."

Women who have borne children say wearing a belly feels uncomfortably similar to the real thing. New mother Kay Gramling, who teaches marriage in the family and child development at Hopewell High School, Hopewell, Va., donned the garment with her pregnant condition fresh in her memory. "The movement is not the same type you would get from an active baby," says Gramling, "but you get a feeling that might be like an elbow. It does give you the sensation that something is moving inside you."

Fathers in birthing classes who volunteer to wear the belly seem to be its biggest fans: Tom and Becky DeCarlo of Williamsburg, for example, had a son in July. "I wanted to participate in Becky's pregnancy as much as I could," says Tom DeCarlo. "I was eating what she ate. We were reading all the books. As much as I was trying to help Becky out, {wearing the belly} was the real eye-opener.

"I had to pick up all these magazines scattered on the floor. When I first bent over, I almost fell forward. I had to get down on my hands and knees. I wore it for the whole two-hour class. When it came off it was a relief. This is something every father, every partner should put on."

Doris Walsh, birthing instructor at Columbia Hospital for Women, has a different point of view. "From what we've read," she says, "we think it promotes strong negative feelings about pregnancy. Pregnant women need to feel good about their bodies and from what we've heard it is a distorted, comical and negative way to view the changes that occur.

"We encourage women to feel good about their body image," says Walsh. "We teach them ways to shift their weight, and we encourage an acceptance of her body during this time."

There is an undecided contingent as well: "We don't have one {a belly}," says Lynette Philip, birthing class preparation instructor at Georgetown Hospital, "although we have talked about it. This is a very conservative community, and people aren't likely to want to try new things.

"Often these devices are just trends," she says. "They come and go. We work with many people who have postponed having a child and aren't as interested in the latest trends."

Fad or not, designer Ware says the belly has a large number of fans, including one who told her: "Now if you can just invent something that simulates labor we'll be set."