Families who call the Outdoor Nursery School in Chevy Chase this month looking for an opening for their preschoolers in September's classes will be shocked to learn that they are too late even for the school's waiting list.

The classes for 2 1/2-year-olds and 3-year-olds will be filled almost entirely with siblings of children who have attended the school.

And parents who inquired before February will be getting letters soon asking if they want to be put on the waiting list for September.

"We definitely are in a baby boom," said Outdoor Nursery School director Barbara Hutchinson. Montgomery County public schools demographer Bruce Crispell agreed. "In 1983, 8,468 births were registered in Montgomery County. That jumped to 9,048 in 1984 and 9,944 in 1985 -- an increase of almost 1,000 babies in one year," he pointed out.

That is manifested in the population crunch in nursery schools ranging from tiny parent-run co-ops to old-line church-sponsored groups and new for-profit schools.

Five days a week, 9 to noon, classes for 2 1/2- and 3-year-olds "are the hottest spots in town," according to Kelcey Klass, director of the Bethesda Country Day School. Marybelle Waldroff, director of the Chevy Chase United Methodist Church Nursery School, said, "The mother may be working, and it may be an au pair or the housekeeper who picks up the child at noon, but a traditional half day of nursery school is still what a lot of parents want for their children."

What parents are doing, as word gets around that preschool spaces are at a premium, is the same thing parents have been doing with college-bound youngsters for years -- they are applying to more than one school. "I definitely wanted to be in the co-op in my neighborhood, but I put my child on two lists just to be sure," said Judy Borzilleri, president of Cedar Lane Cooperative Nursery School in Bethesda.

Multiple applications are a problem, preschool directors freely concede. "It's a self-fulfilling prophecy," said one director who asked not to be identified. "The more applications a school receives, the more rejections it has to send out, and the more youngsters go on waiting lists."

Some schools ask for a deposit for a child on the waiting list. Other schools charge a nonrefundable application fee. Almost all prechools require full or partial payment of tuition during the summer for children who have been accepted for the fall.

For preschools in the densely built Bethesda-Chevy Chase area, part of the problem is that they have little room to expand. The Chevy Chase United Methodist Church Nursery School on Connecticut Avenue, for example, uses space in the church and has no way to enlarge its facilities.

The Outdoor Nursery School, which operates in a rambling old house that has been designated a national historical site, cannot expand physically but has tried another solution -- offering half-day preschool classes in the afternoons.

Other schools offer a range of time and age options -- three-day mornings and afternoons for 3-year-olds and five days a week for fours, for example.

Not every parent is buying into the preschool scene. One is Marla Simons, the mother of a 19-month-old who is at home with her child. "We do a lot of things together, and we're in a play group, but you hear so much about preschool, and I wonder, is it really necessary?" she said.

On the other hand, Outdoor Nursery School's Hutchinson said she sometimes gets calls from parents from the maternity ward of a hospital, and many nursery school operators say they are frequently visited by parents with babes in arms.

"We think it's fine that they are looking for information early on, but we have made a rule," Hutchinson said. "We are not taking applications more than a year ahead."