"I guess you're going to have a hard time believing this . . . . "

So said Keith Andrus yesterday, each time he called another friend or relative to announce what happened in the bathroom of his family's home in Wheaton Thursday morning.

"I still have a hard time believing it myself," he said from his wife's bedside at Holy Cross Hospital in Silver Spring.

"Debbie had a baby!"

A baby? But Debbie wasn't . . . . "

Oh, but she was. Debbie Andrus, 28, who describes herself as a chubby but not obese woman, was pregnant, and had been for eight months. Strange thing was, she said she didn't know it until 4:30 a.m. Thursday. As she sat in the bathroom of her Wheaton home, enduring labor pain that she mistook for severe constipation, Andrus said, she suddenly gave birth to her first child, a 4-pound, 12-ounce boy.

"I had no idea at all," she said from her hospital bed yesterday, delighted by a surprise arrival that one Holy Cross obstetrician called "uncommon, but certainly not unheard of."

By the time she stopped screaming in the bathroom, her husband had lept from bed and dashed to her side. Keith Andrus looked down at his wife of eight years sitting on the toilet, their first child in her trembling arms.

"I thought I was in a movie," he said.

Overcoming shock, they wrapped a towel around the infant, who had entered the world with a splash in the toilet bowl, and the unexpectant dad then called for an ambulance.

Christopher Keith Andrus, premature but healthy, was in good condition at Holy Cross last night, a spokesman said. "We're just so very, very happy," said Debbie Andrus.

The couple, who own and operate the Gude Video store in Rockville, expect to bring Christopher home next week.

The boy's father, meanwhile, has been on the telephone.

"I guess you're going to have a hard time believing this . . . . "

Debbie Andrus, a Montgomery County native and Wheaton High School graduate, stands five feet tall. When Christopher was conceived, she said, she weighed about 120 pounds.

She had stopped using birth control pills in August 1986. She had become dissatisfied with her gynecologist and stopping seeing the doctor, she said, but never got around to finding a new one. Her prescription soon lapsed.

In place of the pill, she said, she and her husband switched to the rhythm method of contraception.

It appears they fell out of step last April.

"I started gaining weight," said Andrus, who eventually added about 20 pounds. "I was eating, which I do very well, unfortunately. I've always been the type whose weight goes up and down -- and up."

Looking back, she recalled feeling Christopher kicking inside her, though she had no way of recognizing the sensation. "I thought it was acid indigestion or something," she said.

And she appeared to continue to menstruate. "It's called 'placental surge,' " said Dr. Craig Dickman, a Holy Cross obstetrician, who said that as many as 40 percent of pregnant women have some cyclical bleeding during their terms.

Dickman said such surprise deliveries are more common among unwed teen-agers, who sometimes are so frightened by the prospect of parenthood that they convince themselves they're not pregnant.

"She's the first adult I've ever come across like this," he said.

Andrus said she began having stomach pain about 1 p.m. Wednesday, while working with her husband at their store. She drove home to Grandview Avenue about 6:30 and fell asleep.

By the time Keith Andrus got home about 9:30, she said, the pain had gotten worse.

They went to bed after midnight. The pain was worse still. Feeling constipated, she took two teaspoons of a laxative about 4:30 a.m. and walked into the bathroom.

"I just sat down on the toilet and pushed real hard, hoping something would just unclog," she said. "And then . . . ."