New technology that allows doctors to freeze embryos for future implantation in a womb is progressing rapidly, but the United States has yet to face the ethical issues it raises, a commentary in The New England Journal of Medicine warns.

"Lengthening the embryo's external existence increases its independence from its parents," write Clifford Grobstein and two colleagues from the University of California at San Diego. "For example, the embryo will presumably not undergo substantial biologic changes over a period of 30 years, but the egg donor will be aging and losing the capacity to receive the embryo back . . ."

Only a handful of births -- none in the United States -- have been achieved with frozen embryos. The advantage over other test tube baby techniques is that several eggs can be removed and fertilized at once, then stored, requiring only one surgical procedure on the woman in an otherwise infertile couple.

But doctors are then left with fertilized, frozen embryos whose legal status is unclear. If one embryo is implanted successfully, there can be others left over -- embryos that researchers might want to use. The authors suggest:

"On thawing, embryos should be returned to the uterus of the donor . . ."

"Thawed embryos should be transferred to a nondonor only with the consent of the donor" and a review by an ethics panel.

"Frozen embryos should be kept in storage for not more than five years or until the establishment of relevant public policy."

Just when such a public discussion might come about is unknown. But the doctors warn: "Though the embryo is frozen and thus for the moment inanimate, both the embryo and the issue of its status remain very much alive."