For at least three hours a day, six days a week, 14-year-old Genna Weiss strides carefully out to the edge of a diving board more than 30 feet high and considers the "what ifs."

What if she slips? What if she falls? What if she fails to clear the board? What if she can't become vertical in time? The possibilities for failure, injury and yes, even death, seem infinite.

"There is fear in diving at all times," said Weiss, of Silver Spring. "I've even talked to the great divers, people like Greg Louganis, and they are still fearful. There's a lot to consider up there, a lot to think about.

"There's always that 'if.' "

Although the daily contemplation of one's mortality at such an age seems a terrible burden, such are the obstacles confronting Weiss in her development as one of the premier divers in the world. The junior world champion in 1984 and 1985 off the one-meter board, Weiss was the youngest diver ever to qualify for the senior national championships last year or to include a forward 1 1/2 dive with three twists in her repertoire.

Weiss, ranked 13th among U.S. divers, is expected to challenge for a spot on the 1988 Olympic team.

Those accomplishments have earned Weiss nationwide media coverage, but also have forced an accelerated maturation process. The braces and mop of blond hair indicate a child, but the poise and self-confidence say otherwise.

"Diving is my life," Weiss, who turned 14 yesterday, said with a shrug. "I was born with this ability, and I have an obligation to use it. Sometimes I feel like I'm one of the best, but then I realize nobody reaches their potential at 13."

Weiss and her younger siblings provide a strong argument for the inheritance of athletic ability. Her father, Greg Weiss, was an Olympic gymnast in 1964. Her mother, Margie, was on the U.S. national team four years later. The Weisses have operated the MG gymnastics club out of their home since 1970 and have churned out a stream of world-class competitors.

But their children are their top products. Their 12-year-old daughter, Geremi, is intermediate Eastern figure skating champion. Their 9-year-old son, Michael, is expected to compete in the South Atlantic figure skating in 1986.

"We pride ourselves on creating a competitive environment," said Greg Weiss. "We don't push our kids. We 'love' their successes out of them. Genna knows that if she trains hard and gets as good as she can, then no one can beat her."

"One thing I learned from my parents is how to compete," said Genna, who stopped counting her trophies and medals two years ago at 150. "I feel lucky to have parents that are talented. They taught me how to win."

Weiss is small and graceful, capable of leaving even a springboard silently. Her entrance into the water is just as quiet. "She has an uncanny ability to 'rip,' " said John Wolsh, her coach of seven years. "She can enter the water with virtually no splash at all."

Weiss was supposed to be a gymnast, and she trained for two years with her parents. But after deciding she wanted to do something different as a 6-year-old, she turned to diving. Weiss won the first meet she entered six months later. "I was so proud," she said. "I wore that medal around all day."

Her pride still is evident in her dedication to the sport. Weiss, who will be competing Friday and Saturday in the Potomac Valley Junior Classics at Wilson High School, trains for three hours every day after school at Wilson in the District. Twice a week, she goes to Annapolis with Wolsh for two more hours of tower diving.

Although that commute keeps Weiss from getting to bed before 11 p.m., it beats the weekly drive to Philadelphia that was required last year before pool time could be secured closer to home.

Wolsh doubts that the countless hours on 40-foot platforms or in practice harnesses can be credited for Weiss' success. "The key to Genna is not physical," he said. "It's in between her ears. Most people that lose beat themselves mentally or spiritually, but not Genna. She won't let herself make mistakes."

But just when Weiss seemed in complete control of her future, she began to grow. "She's grown four inches since summer and gained at least 10 pounds," said Wolsh. "You could see physical differences in her on a weekly basis."

"I wouldn't let it stop me, though," said Weiss. "I kept training hard and adapting to my body. Most people lose their balance as they grow older, but I wouldn't let it affect me."