Doctors have issued a stark warning to women choosing to delay pregnancy: Don't count on freezing eggs to offset a ticking biological clock just yet.
The new guidelines issued from fertility specialists conclude the procedure remains highly experimental even though it is increasingly offered.
The recommendations are a dash of cold water for a field that proponents believe is slowly coming of age.
But with perhaps 500 births from frozen-and-thawed eggs worldwide, compared with more than 200,000 from frozen embryos, it is a nascent technology.
Now scientists are tweaking techniques to try to improve the odds that a woman who has her eggs removed and frozen will bear healthy children from them years later.
Until then, the American Society for Reproductive Medicine issued a list of warnings that it says women considering the expensive procedure must be given to ensure they make an informed choice.
"There are far fewer published outcomes for ... thawed previously frozen eggs than many might believe," cautions Dr. Marc Fritz of the University of North Carolina, Chapel Hill, who led the ASRM's deliberations.
Sperm routinely are frozen. So are the extra embryos of couples undergoing infertility treatment, for later pregnancy attempts.
Egg freezing evolved more recently, as a means of preserving fertility for young women or girls diagnosed with cancer or other serious illness that would destroy their ovaries. It also is marketed as a way to help women postpone conception, as fertility rapidly declines beyond age 35.
The problem: Eggs contain lots of water, making them more vulnerable to freezing and thawing. Ice crystals can form during either process, injuring or even destroying the eggs.
That makes it far from certain that women who spend more than $10,000 to freeze eggs will have usable ones years later, the reproductive society concluded — calculating a 2 percent to 4 percent chance of a live birth for every thawed egg.
Still, patients with cancer and other illnesses may be appropriate candidates for egg freezing, as long as they're thoroughly warned — because they have no other options, the guidelines say. But the reproductive society said it was too soon for egg freezing to move into the mainstream for healthy women.
Anyone considering it must be told that:
_There are side effects from the ovary-stimulating drugs used to retrieve eggs.
_There's a possibility that none of the stored eggs will survive.
_Women who freeze eggs before age 35 will likely never need to use them.
Most centers that freeze eggs agree that it remains experimental, says Dr. Richard Paulson, director of the University of Southern California's in vitro fertilization program. His own center freezes eggs for three to four healthy women a month.
"That doesn't mean that it shouldn't be made available," adds Paulson, who said in a study of 18 pregnancy attempts with frozen eggs in his center, 10 succeeded. "I inform them very carefully that this may be completely unnecessary ... that the technology may be so advanced five years from now there'll be something entirely new."
But the "experimental" designation obligates fertility centers to track their successes and failures, Paulson adds — so scientists can quickly settle such outstanding questions as whether the technology is safe.
Last week brought reassurance on that front. Chicago researchers found no increased risk of birth defects among published reports of 197 births through July 2006. Separately, doctors at a Bologna, Italy, fertility clinic had similar findings among 105 babies born there.
The bigger question is how to best freeze and thaw eggs, to improve pregnancy attempts. Some eggs are slow-frozen, over an hour or more. Others are flash-frozen by dunking them in liquid nitrogen, a process called vitrification.
University of Michigan researchers compared the techniques, and found slightly more flash-frozen eggs survived thawing. And it took 21 vitrified eggs to generate a pregnancy, compared with 45 slow-frozen ones.
California's Paulson cautions that the jury's still out on which method to use. His own consumer advice: Ask fertility specialists, "How many babies have been born from this technique in your hands?"
— Associated Press