Colleen Convery did not want her baby to suffer the same fate as her three brothers. All of them died of Duchenne muscular dystrophy, an aggressive form of the muscle-wasting disease that usually kills its victims before their 30th birthdays.

The best way to protect her child from that fate, she learned, was to give birth to a girl.

"There was a 50-50 chance that if I had a son, he would have muscular dystrophy, too," said Convery, 36, of Rutland.

But, a controversial advance in the screening of human embryos saved Colleen and her husband, Jerry, from that terrible risk. Last year, fertility specialists at Brigham and Women's Hospital in Boston fertilized eggs for the couple in their lab and, based on the resulting embryos' sex chromosomes, selected only females to implant in Colleen's body.

They now have a healthy, 7-month-old daughter, Erika. She may be a carrier of the disease as was her mother, but Duchenne muscular dystrophy is one of a number of diseases that affects only the health of males.

"I had always thought that if I got pregnant, I would have a child with an illness," Convery said. "I feel very, very lucky."

The procedure used to help the Converys, called preimplantation genetic diagnosis, allows prenatal testing months earlier than conventional tests like amniocentesis - indeed, before a pregnancy even begins. Doctors can test a single cell from an eight-cell embryo that is just days old to determine, among other things, whether it is a boy or girl - crucial information for genetic diseases that afflict just one gender.

So far, more than 700 healthy babies have been born worldwide after undergoing the procedure, according to the International Working Group on Preimplantation Genetics. And the number is growing rapidly.

But, as is the case with many advances in reproductive medicine, gender selection using this diagnostic method has been under increased scrutiny because of what it offers to couples, unlike the Converys, who have no medical need to preselect the sex of their child. Some fear the test's impact if it became widely available in societies where families prefer boys over girls.

"Certainly, this seems to be the first step toward picking characteristics of a child," George J. Annas, chairman of the Health Law Department at Boston University, said of nonmedical use of the procedure. While he cautioned that the scientific community is still many years away from creating "designer babies" in the lab, it is important that the ability to pick the sex of a child be viewed broadly. "It has societal implications. It is not just an issue between a couple and their doctor."

Identifying the gender of embryos came to the forefront in the 1990s as a new way to slow the transmission of rare diseases that are carried on the X-chromosome that mothers pass on to their sons, such as Duchenne muscular dystrophy and Hemophilia A. Because boys have just one X-chromosome, they are more vulnerable to genetic abnormalities in the X. For girls to get Duchenne muscular dystrophy, both their X-chromosomes would have to carry the defect, which is a virtual impossibility.

But preimplantation genetic diagnosis, or PGD, which is available at only about 50 clinics and hospitals worldwide, has also been used to detect other abnormalities in the chromosome structures of embryos that are not related to gender, according to Dr. Catherine Racowsky at Brigham and Womens Center for Reproductive Medicine. Many of these abnormalities, if undetected, would likely have resulted in miscarriages; others, would cause mental retardation.

There are also a growing number of single gene disorders, such as Marfan syndrome, Gaucher disease, Tay-Sachs disease, sickle-cell disease and certain cystic fibrosis mutations, that have been detected using a more sophisticated method of PGD. This list is expected to grow as researchers learn more about the composition of the human genome.

Hospitals and clinics that offer PGD say the tests have brought a new group of patients to them - couples who have not experienced infertility, but rather who fear passing on a genetic disorder that has affected their family.

Deborah Fayne, 34, of Stoughton was worried before she ever became pregnant that she might not be able to have a child because of a condition prevalent on her husband's side of the family known as a Robertsonian translocation, a known cause of Down syndrome. When a blood test showed that her husband carried this as well as a second chromosomal abnormality, she knew her chances of bringing a healthy baby to term were slim.

She and her husband, Adam, sought help at the Fertility Center of New England in Reading. Once they began the in-vitro-fertilization process, they learned that, of the 22 successfully fertilized eggs, only two were considered to have a normal balance of chromosomes. Those two embryos were implanted. "One attached itself and that's Noah," she said, referring to their now 10-month-old son.

"I know I didn't have it in me to go through 13 miscarriages," she said. "Nothing is a guarantee, but what gave us the edge was that at least we knew that the [embryos implanted] were normal."

The Fertility Center of New England is one of a growing number of private clinics that have a PGD program, currently limited to couples with genetic concerns. According to the center's laboratory director, Michael Lee, about 40 couples have been helped through this program, which dates back to 1998.

By contrast, Boston's teaching hospitals have moved more cautiously into preimplantation genetic screening of embryos. Brigham and Women's Hospital, in conjunction with Massachusetts General Hospital, began its PGD program last year, accepting patients with very specific genetic concerns. New England Medical Center's Division of Genetics reported its first successful PGD-aided birth in June 1999.

In part, because many of the leaders in applying PGD in the United States are private fertility clinics, some fear the technique - which adds about $3,500 to a roughly $10,000 in-vitro-fertilization procedure - soon will be used to help couples who would simply prefer to have a boy or a girl.

"This is such a competitive part of medicine," said Jeffrey P. Kahn, director of the Center for Bioethics at the University of Minnesota. "All the decision-making is left to the clinics. All it takes is one to offer gender selection [for nonmedical needs] and they will all follow suit."

While there is no regulatory body that governs the use of techniques that aid reproduction, the nation's practitioners look to the American Society for Reproductive Medicine for ethical guidance. The society reiterated its cautious position on gender selection, stating that "the initiation of [in-vitro fertilization] with PGD solely for sex selection holds even greater risk of unwarranted gender bias, social harm and the diversion of medical resources from genuine medical need. It therefore should be discouraged."

But at least one leading fertility specialist argued that the society is sending mixed signals on the ethics of choosing embryos for implantation based on gender.

Already, a number of clinics in the United States and abroad can identify sperm samples most likely to produce either a boy or a girl. The Society for Reproductive Medicine's ethics committee in May said that physicians should be free to offer, on an experimental basis, these "sorting" or "spinning" services, which are reportedly only 70 to 85 percent effective, to parents wishing to balance the gender makeup of their families. The committee added that patients should be informed of the risks of failure, among other things.

If "spinning" is alright, critics asked, then why isn't the more accurate PGD test?

"We felt very strongly that this wasn't a logical or ethical viewpoint because nowhere in medicine would we ever offer an inferior technology [sperm sorting] when there is a superior one [PGD]," said Dr. Norbert Gleicher, chairman of the board of a network of private fertility clinics in Chicago and New York.

Gleicher initially announced plans to offer the sex selection service to patients using PGD even if they din't have a medical need for the information. But, when the head of Society for Reproductive Medicine objected, Gleicher agreed to hold off on offering the service. He has asked the organization, however, to take up the issue at their meeting in January.

Gleicher argues that allowing parents to choose their babies' gender would not lead to sex discrimination - in the developed world, at least.

"In the majority of cases, these are couples where the choice is not between a boy or girl, it is between no more pregnancies or trying one more time to get a kid of the other gender," he explained in a telephone interview.

Many others disagree.

"Ultimately, gender selection will reinforce gender discrimination," said Evelyne Shuster, a medical ethicist at the University of Pennsylvania and a former member of the ethics committee of the American Society for Reproductive Medicine. Shuster said this is particularly true in countries "where the main human rights problems are inequality and discrimination."

"Gender is not a disease so, therefore, there is no reason to correct or address it," she said. "There is no reason to try to have one gender and not the other. . . . To have a balanced family is not an ethical principle."

In addition, many draw an ethical distinction between attempts to increase the likelihood of a boy or girl using sperm-sorting techniques that occur before an embryo is formed and the near- certainty of PGD gender selections that involve discarding embryos of the undesired gender.

"For me, personally, I would feel like we were playing God a little bit," said Dr. Vito Cardone, medical director of the Fertility Center of New England, referring to using PGD for sex selection when not medically warranted. "We're here to help when nature is not doing things right. We're not here to take nature's place."

Several Boston-area doctors in the field cautioned that more needs to be known about PGD before it is endorsed for widespread use, even for medical needs.

"This intervention is not completely benign," said Dr. Elizabeth Ginsburg, an embryologist at the Center for Reproductive Health at Brigham and Women's. "We are still looking at the [successful] implantation rates of embryos that have been tested."

Preimplantation genetic diagnosis, or PGD, which is available at only about 50 clinics and hospitals worldwide, has also been used to detect other abnormalities in the chromosome structures of embryos that are not related to gender, according to Dr. Catherine Racowsky at Brigham and Womens Center for Reproductive Medicine. Many of these abnormalities, if undetected, would likely have resulted in miscarriages; others, would cause mental retardation.

By contrast, Boston's teaching hospitals have moved more cautiously into preimplantation genetic screening of embryos. Brigham and Women's Hospital, in conjunction with Massachusetts General Hospital, began its PGD program last year, accepting patients with very specific genetic concerns. New England Medical Center's Division of Genetics reported its first successful PGD-aided birth in June 1999.

While there is no regulatory body that governs the use of techniques that aid reproduction, the nation's practitioners look to the American Society for Reproductive Medicine for ethical guidance. The society reiterated its cautious position on gender selection, stating that "the initiation of [in-vitro fertilization] with PGD solely for sex selection holds even greater risk of unwarranted gender bias, social harm and the diversion of medical resources from genuine medical need. It therefore should be discouraged."