Four-month-old Rachel Jessica Wolff is healthy. For her parents, that is cause for daily celebration.

Neil and Ann Wolff of Mountain View are both carriers of Tay-Sachs, a fatal genetic disease that affects a disproportionate number of Jews of Eastern European descent. With the help of a relatively new technique called pre-implantation genetic diagnosis, however, the Wolffs now have a Tay-Sachs-free baby.

What's more, the smiling, fair-haired infant is not even a Tay-Sachs carrier.

"The real beauty of this technology is that it holds the promise to completely eliminate this disease," her mother says.

When both parents are carriers, there is a 25 percent chance of conceiving a child with Tay-Sachs, which attacks newborns' central nervous systems. Before Rachel's conception, the couple aborted two fetuses shown by amniocentesis to be afflicted.

"We were very much at the end of our rope," says Ann Wolff, a 38-year-old social worker. "We were overwhelmed with the loss and grief and disappointment. We were really ready to give up on having children."

Instead, they turned to pre-implantation genetic diagnosis, a procedure they had tried with no success after their first terminated pregnancy. Performed at only a handful of clinics around the country, the procedure involves developing embryos outside the mother's body and screening them for certain genetic diseases before implanting them in the uterus. Fewer than 100 children worldwide have been born with the aid of the technique.

Last year, at the Jones Institute for Reproductive Medicine in Norfolk, Va., doctors put Wolff on fertility drugs to stimulate egg production. After extracting four eggs and fertilizing them with her husband's sperm, they waited until each of the embryos grew to comprise eight cells. They then took a single cell from each embryo and used the latest technology to analyze the DNA and determine which embryos were Tay-Sachs-free.

As it turns out, all four developing embryos were found to be free of the disease, with two registering as carriers. To increase Wolff's chances of becoming pregnant, all four embryos were implanted in her womb.

According to pre-implantation genetic diagnosis specialist Susan Gitlin, women undergoing the procedure typically have more than one embryo implanted. Gitlin, who is an instructor of obstetrics and gynecology at Eastern Virginia Medical School, pegs the chances of a multiple birth at 10 to 15 percent.

The Wolffs were willing to take the risk.

"After all the losses, twins would have been a blessing for us," Ann Wolff says. In the end, only one embryo thrived. Born Nov. 5, Rachel is happy and placid, an "easy baby," her parents call her.

There is no treatment or cure for Tay-Sachs, named after two doctors who identified the syndrome in the late 19th century. Most victims of the disease, which destroys the brain and causes the body to shut down, die by age 5.

While a large percentage of carriers are Ashkenazic Jews, other groups with high numbers of carriers include Cajuns and French Canadians. Sephardic Jews carry the gene at about the same rate as the general population — one in 200.

Pre-implantation genetic diagnosis takes about two weeks and costs approximately $10,000. Brittany Nicole Abshire of Louisiana, the world's first Tay-Sachs-free baby born using the procedure, is now nearly 4 years old.

A couple in which both partners are carriers has a number of other options when it comes to having children. They can adopt. The woman can be artificially inseminated. They can, as the Wolffs did, have their pregnancy monitored by amniocentesis and abort if the fetus is found to be affected.

Many couples choose the latter method, as it is cheaper and less elaborate than pre-implantation genetic diagnosis. But some abortion opponents oppose it, since affected embryos are discarded.

For the Wolffs, the decision to terminate two pregnancies was painful but necessary. "I feel we've lost two kids," says Neil Wolff, a 39-year-old attorney. Still, "to watch a child die slowly and horribly over the course of a couple of years…no, thank you."

Neil Wolff first learned he was a Tay-Sachs carrier at age 15, when his Jewish youth group in Atlanta participated in a free testing program. Years later, while a law student, he had his blood tested again, hoping for a different result. "I had not liked the answer," he says.

In 1992, when he met his wife-to-be through a Jewish Bulletin personals ad and the pair started dating, he told her of his carrier status. She, on the other hand, had never been tested for the disease. "It was not something I thought about," she says. "I didn't know anyone who was a Tay-Sachs carrier. It was never an issue."

She refused to get tested until after the couple became engaged in early 1994. "She didn't want the answer to the question to affect our decision about whether or not we loved each other and wanted to spend our life together," her husband recalls.

Given that an average of one in 25 to 30 Jews of Eastern European descent tests positive for being a Tay-Sachs carrier, the odds weighed heavily in Ann Wolff's favor. But a week after the joyful occasion of the couple's engagement came the bad news: She, too, was a carrier of the fatal disorder.

The news shocked them both — and spurred Ann's family members to get tested. "We found out my father was a carrier, and two of my siblings were carriers," she says.

The Mountain View couple, meanwhile, faced a whirl of emotions. They felt anger and denial. They shed tears while clinging to the hope of a smooth path to parenthood.

Through it all, Ann Wolff says, the pair embraced a basic philosophy: "Our love is so strong, we will work this out. Whatever happens, we'll deal with it, which indeed we've been able to do."

The couple, in fact, plans to have another child as soon as possible, though they're not yet sure which path they will take. "We will make up our minds over the next couple of months whether we will go back to the Jones Institute or roll the dice again," Neil Wolff says.

But while the notion of rolling the dice and possibly having to terminate another pregnancy once seemed too painful to bear, it now strikes them as tolerable.

Their emotional odyssey, after all, has landed them the top prize. With Rachel at home, her father says, "We've got something to hold onto."