Seven years ago, when she was 13, Jeanne Gaetano first heard of abortions. It became her cause. She began demonstrating against abortion clinics at 15, met her husband in a protest at 17, and served her first prison sentence for a protest sit-in at 18.

Yesterday 20-year-old Gaetano, who dropped out of college to devote more time to the protests, walked in the Alexandria city jail, taking her 7-month old son behind bars with her to serve a 24-hour sentence she received as a result of one of her almost weekly demonstrations.

"He was with us when we were arrested," she said as she and her husband moved toward the jail with their blue-eye boy wrapped in a white, terry cloth jump suit. The child thus became the first infant to be housed in the city's 155-year-old white brick jail house.

The Gaetanos, who live in suburban Seabrook, had refused to enter the jail to serve their sentence without the child, who is being breast-fed. "We were not about to let the law decide when to wean him," said David, as he pushed a collapsible playpen into the jail behind his wife and son.

Yesterday's events, which attracted three television crews and wire service reporters to the jail, were as much a testimony to the durability of the anti-abortion movement in the Washington area as it was to the commitment of the Gaetanos. The protests like the one in which they were arrested in September 1979 have continued on an almost weekly basis at abortions clinics throughout the area. Their movement rarely attracts much attention these days, but it can count on support from a core of dedicated followers who say they will not compromise on the issue.

"It was legal to kill Jews in Germany, too," said David slowly as he tugged his reddist beard and explained the night before why he was going to jail. "The Nuremburg trials have proved that that was no defense, the action was murder, just as abortions being performed today are murder."

Gaetano, who will be 30 today, said he regreted having to go to jail this weekend because it's the eve of the general elections and he has been active in the U.S. Senate campaign of Democrat Edward T. Conroy, who also is opposed to abortions. "A day now is like a week any other time in the campaign," he said.

David, who works for an antiabortion newspaper, and his wife were ordered to jail last year when they were convicted of illegally entering an Alexandria abortion clinic during a demonstration. They had appealed the decision to Virginia's highest court, but after they lost their appeal last month they were ordered to serve their one-day sentence this weekend.

The Gaetanos were driven to the jail yesterday by another outspoken abortion opponent, Mary Ann Kreitzer, who was arrested with them in the same protest, but who served her one-day sentence two weeks ago. "It was worse than I expected," Kreitzer recalled as she watched the Gaetanos climb the steps and disappear into the jail.

Alexandria police officials said that the Gaetanos were assigned to the isolation unit on the first floor of the jail where three cells had been "sanitized" in preparation for the infant. The mother and her child, named John Paul in honor of the Pope, were placed in adjoining cells and David was locked up nearby.