WASHINGTON — As the number of women delaying motherhood continues to rise, many fertility clinics are starting to offer a new service that allows them to freeze some of their eggs to buy more time on their biological clocks.

At least 138 clinics are freezing and banking eggs — more than double the number three years ago, according to one count. Hundreds of women have frozen their eggs so far, and the numbers are increasing dramatically, experts say.

"I think we're sitting at that tipping point between technology that is quasi-experimental and tipping over into fairly widespread use," said David Grainger, president of the Society for Assisted Reproductive Technology, which represents fertility clinics. "It's one of the most exciting areas in our field right now."

The popularity of egg-freezing is driven by advances that have boosted the chances of having a baby using thawed eggs and intensifying demand from childless women in their 30s. But the trend has sparked intense debate about whether the technology is ready for wider use and whether society is ready for its impact.

Proponents say egg-freezing could save many women from the wrenching disappointment of running out of time to bear their own children, marking a profound step toward freeing them from some of the constraints imposed by biology.

"In the same way the birth control pill gave women in the '70s a whole new set of options, I think egg-freezing can do the same with this new generation of women — giving them more control over their fertility and giving them more options," said Christy Jones, of Extend Fertility Inc., of Woburn, Mass., which markets egg-freezing for clinics.

But some experts say more research is needed to know how well egg-freezing works and whether babies born from frozen eggs grow up healthy. Although there is no evidence that egg-freezing poses risks for offspring, far too few babies have been born to know for sure. The first pregnancy from frozen eggs was reported in Australia in 1986, and there is no reliable information about the outcome. There were very few such births until recently, and only 300 to 600 such babies have been born worldwide. Researchers are just now making plans to systematically track the health of these babies.

Beyond whether egg-freezing is safe and effective, critics say the prospect raises a host of societal issues, including: Is it in the best interests of women or their children to delay parenthood to the 40s or beyond? Is the technology further transforming pregnancy from a natural process into an expensive high-tech project? And does it let society off the hook for failing to create family-friendlier workplaces for younger women early in their careers?

"This opens the gate to all kinds of questions," said Evelyne Shuster, a medical ethicist and philosopher who has advised the American Society of Reproductive Medicine, which represents reproductive health specialists. "We need to address these questions before we let any of this go further than it already has."

Fertility clinics have long worked with frozen sperm and embryos, but freezing eggs has proved much harder. In recent years, however, scientists, motivated primarily by the goal of helping women left sterile by cancer treatment, have honed their techniques, experts say. Some eggs are still lost in the process, and success rates vary among clinics and by women's ages. But advocates say women now have a better chance of having a baby with eggs they froze in their 30s than with their own unfrozen eggs in their 40s.

"I have traditionally been very conservative," said Michael Tucker, an embryologist at Georgia Reproductive Specialists in Atlanta and Shady Grove Fertility in Rockville, Md. "But I'm ready to throw caution to the wind and say, 'We're ready to do this.' "

There are no official figures on how many clinics offer egg-freezing or how many women are using it. One advocacy group for cancer survivors, Fertile Hope, recently surveyed 430 clinics and found that 138 were providing the service, up from 58 three years earlier. When the group queried those centers further for The Washington Post, most said they were doing it for cancer patients and for extending fertility, and they reported having done more than 500 egg retrievals for women delaying motherhood. Extend Fertility, which recently expanded to a sixth city, says it has signed up more than 200 women in the past three years.

Egg-freezing costs $9,000 to $15,000 per attempt, plus $350 to $500 a year to store the eggs. Although some women opt to freeze their eggs to avoid moral dilemmas created by standard infertility treatments that frequently result in leftover embryos, most are choosing it to allow later childbearing.

Karen Soika decided to freeze her eggs at 38 after a relationship ended, she decided to go to medical school and she realized that having children anytime soon was unlikely.

"I know I want to be a mom and have a family at some point," said Soika, who lives in New York. "But I can't pursue that at this point. This is kind of like my insurance policy. It gives me the alternative of still having my own child with my own gene pool."

Although agreeing that egg-freezing holds promise, many experts say too few studies have been published to substantiate the success rates that some clinics claim.

"Currently available evidence does not validate the assumption that if you freeze your eggs now, your chances of a successful pregnancy will be better than your chances using your own fresh eggs at that point," said Marc Fritz, a University of North Carolina reproductive endocrinologist. He spoke on behalf of ASRM, which recommends limiting egg-freezing to cancer patients and research studies.

Fritz and others experts worry that egg-freezing might lull women into assuming it will make it easy to have children in their 40s. A woman's chances of conceiving are still much better in her 20s and 30s.

"Our concern is women may be making life decisions viewing this as effectively having purchased an insurance policy. The success rates don't remotely support that," Fritz said.

But other experts say confidence in egg-freezing is rising rapidly, especially for women who freeze their eggs in their early to mid-30s. For example, a woman who freezes her eggs when she is 32 might have close to a 40 percent chance of having a baby with those eggs later, according to a recent analysis of published data by fertility specialist Kutluk Oktay, of Cornell University.

"Every month we are getting the news of new pregnancies and further success," said Oktay, who figures egg-freezing makes sense for women in their 30s who think they are at least three to eight years from trying to have children. Oktay, who heads an ASRM committee reviewing the specialist group's policy, expects the organization will relax its objections.

But even if the process turns out to be safe and reliable, some say society should instead make it more practical for women to have babies when it is easier and safer, when they have more energy and are more likely to live long enough to play with their grandchildren.

"It seems like we're finding a solution in high tech that asks women to bear the cost of a socially created problem — a problem we could correct if we had a family-friendly work environment," said Mary Shanley, a Vassar College political science professor. "I'd like to see us focusing on solving that rather than talking women into a risky medical procedure."

Others wonder whether the technology will fuel fundamental demographic shifts in childbearing, with more affluent women tending to have children when they are older — and often at the peak of busy careers.

"We don't know what the cost may be to the children, and we don't know if it's good for society," said Adrienne Asch, who studies family issues at Yeshiva University.

Some worry that egg-freezing is continuing a trend of separating human reproduction from natural rhythms and relationships.

"If anything, I think our society needs to acknowledge the physiological constraints on women's fertility and get back to a more natural timing of childbearing," said William Hurlbut, a Stanford University bioethicist who serves on the President's Council on Bioethics.

Some cite the case of a Canadian woman who has frozen her eggs in order to give them to her 7-year-old daughter, who was born infertile, raising the prospect the girl could eventually bear her own half-sister.

"When we start to muddle up biological categories like mothers and sisters, who knows what the impact will be?" said Robert George, of Princeton University, who also serves on the President's Council on Bioethics.

And because egg-freezing makes it possible to ship eggs around the world, critics fear it could lead to exploiting women in poorer countries, especially since the fertility industry is largely unregulated.

"We really need to consider how this could open the door to a global trade in women's eggs, which could turn out to be a very exploitative situation," said Marcy Darnovsky of the Center for Genetics and Society in Oakland, Calif.

But others maintain those concerns are theoretical, while the needs of individual women are immediate and real.

"What happens now is women who have not met the right partner either decide to go it alone or end up settling for someone," said Rosanna Hertz, a professor of sociology and women's studies at Wellesley College. "This buys more time to find someone who you both love and want to be a parent to your child. This could mean women would no longer be slaves to their biological clocks."