In her high-pressure job as human resources manager for the American Bankers Association, 31-year-old Connie Conley relied on her daily run to relieve pent-up stress. So when she became pregnant last spring, the idea of putting her feet up for nine months was out of the question.

"For me there wasn't any other option," she says, sitting cross-legged on the floor and gently rubbing her eight-months-pregnant belly. Dressed in lime green sweatpants and an oversized T-shirt for her evening's workout at Mothers on the Move in Northwest, Conley asserts, "I had to have some kind of exercise for my emotional and physical health."

Conley is part of a new army of expectant mothers flocking to one of the latest off-shoots of the fitness movement -- prenatal exercise classes designed to keep blossoming bodies in shape throughout the nine-month marathon of pregnancy. Some of these moms-to-be are confirmed workout-aholics who want to stay active. Others seek to avoid physical discomforts of pregnancy such as backache and swelling, and still others want to train for the strenuous task of childbirth and maximize their chances for a speedy recovery.

"Most of the women in our program are career-oriented, 25- to 35-year-old first-time mothers," says Kim Teachout of the Alexandria Hospital's Mama Take Shape program, which began last year with one weekly class and now runs three twice-weekly classes. "They are well-educated and have a keen interest in how exercise will affect their pregnancy and delivery. The most frequent comment I hear is that the classes help them feel more awake and energetic."

Most women begin prenatal exercise classes in their fourth month of pregnancy -- with their obstetrician's permission -- and continue through delivery, Teachout says. Exercises focus on increasing and maintaining strength, endurance and flexibility, and on physical techniques important in childbirth such as breathing, muscle isolation and relaxation.

Researchers are just beginning to probe the effects of exercise on pregnant women, says Laurie Hall, spokeswoman for the American College of Obstetricians and Gynecologists. The ethical problems of experimenting on expectant mothers have limited most research to animals.

"Generally, healthy women with no complications of pregnancy -- such as hypertension, diabetes or twins -- are told that they can safety continue an exercise program they started before pregnancy," she says. "But pregnancy isn't the time to start up a new, strenuous exercise regimen.

"Almost any sport is okay, except those involving sharp blows to the abdomen or a chance of falling. It's not the time to scuba dive or 'go for the burn.' Walking, swimming and mild calisthenics are among the best exercises for expectant mothers."

But even well-conditioned women should take particular care when exercising during pregnancy, she notes, because "generally, you're not as coordinated. Balance changes, ligaments stretch, pelvic bones start to soften."

Physiological changes -- such as increased blood volume and extra weight -- mean that a pregnant woman accustomed to running eight-minute miles may need to slow down to a 10-minute mile pace to achieve the same level of exertion, says Dr. Mona Shangold, director of the sports gynecology center at Georgetown University School of Medicine and coauthor, with her husband Dr. Gabe Mirkin, of "The Complete Sports Medicine Book for Women."

"When a woman is pregnant, her body works harder, even at rest," says Shangold, 37, who is in her third trimester and runs about 10 miles a week at an 11-minute-mile pace -- down from her pre-pregnancy 25 to 30 miles per week at 9 1/2-minute-mile pace. "Just because you could get through a tough aerobics class before pregnancy doesn't mean you should try to keep up at the same speed while pregnant."

Major concerns about prenatal workouts arise from animal studies that indicate aerobic exercise may divert blood away from the uterus and could cause body temperatures to rise to levels potentially harmful to the fetus, she says. On the positive side, she points to another study indicating that the work of labor and delivery is done more efficiently in women who are physically fit.

Shangold's advice: "Don't get overheated, limit aerobic exercise sessions to 30 minutes or less, and avoid exercising to exhaustion." In addition to seeking physical benefits, many women attend prenatal workouts for the social contacts, says Susan Perez, a British-trained midwife who in 1983 started Mothers on the Move -- a group featuring prenatal exercise, nutrition counseling and other services for expectant and new parents.

"Many of these women try to be very professional at work and can't talk about their pregnancy at all," Perez says. "Here they can compare notes with other women and ask questions of the instructors, who are all registered nurses and certified childbirth educators."

Some worry that the trendiness of prenatal exercise classes will spawn a harmful Maternity Macho. "We're very faddish about pregnancy," says Washington journalist Wendy Lesko, coauthor with her husband Matthew Lesko of "The Maternity Sourcebook." "Fitness is in for expectant mothers today, but very little is known about the safe limits of exercise during pregnancy."

One reason the Leskos wrote their book is that they asked five different doctors "How long can a mother-to-be continue to play tennis?" and received five different responses, ranging from "It's okay as long as you don't serve" to "Never play at all."

"Many women today feel guilty if they don't exercise daily," says Lesko, who played tennis until the last trimester of her pregnancy. "But they need to realize that, while exercise can be helpful in pregnancy, it needs to be done in moderation.

"Before they invest $30 in a maternity leotard, they should be realistic about what they can do. It's important to consult their doctor and be sure, if they take a class, that the instructor is qualified to teach pregnant women."

While exercise can help an expectant mother look and feel better, it is not the most important aspect of fitness during pregnancy, says Dr. Donald McNellis, who coordinated a workshop on this topic at the National Institute of Child Health and Human Development.

"The expectant mother who wants to be fit for herself and her baby should stop smoking and drinking," he says. "The benefits of exercise during pregnancy are not yet established, but the benefits of avoiding tobacco, alcohol and drug abuse are clearly there."