The decision of the Rotunda Hospital to freeze zygotes, fertilised human eggs, as part of its infertility treatment programme has inevitably caused controversy. This is despite the conviction of the Master, Dr Peter McKenna, that the practice is in line with the Medical Council's guidelines on fertility treatment.

Dr McKenna uses the term zygote to describe the initial stage of life reached by the fertilised egg, and points to this usage in a textbook on embryology. According to the textbook, after implantation the inner cell mass of the zygote will eventually give rise to the embryo, which in due course becomes a foetus.

The aspect of the procedure which has already attracted the odium of the Pro-Life Campaign is that some of the zygotes may eventually be "thawed without transfer", that is, effectively destroyed.

This will arise only if the couple is unable to fulfil its commitment to have the frozen zygotes implanted, due to separation or the death of one or both partners, or simply because of a failure to return to the hospital.

The freezing of the zygotes is a further development of the Rotunda's Human Assisted Reproduction (Ireland) programme, which has operated in vitro fertilisation (IVF) for more than 10 years. This IVF programme is seen as one of the most successful in Britain and Ireland, with a successful conception in more than one-fifth of attempts.

Under this procedure the ovaries are stimulated by drugs so that a number of eggs are produced. These are then harvested using a needle, fertilised outside the womb by sperm and replanted in the woman's body.

At this stage they are usually made up of four cells.

The number of fertilised eggs produced in this way is usually five or six. However, it would be medically dangerous both for the mother and the potential baby or babies for six to be planted in the womb, given the possibility of up to six babies. So the practice has been for a maximum of three to be planted in the womb, with the remainder planted in the cervix, where they cannot survive.

"The hospital was uncomfortable with this," said Dr McKenna. "It would be better if they could be frozen so that the woman did not have to go back to square one again.

"We looked at the possibility of freezing zygotes and were fully convinced of the medical benefits. We were fairly sure it would be highly acceptable to the patients, provided we replaced the zygotes at some time in the mother.

"We were left with the dilemma of whether this was constitutional. We took legal opinion, and the majority opinion was that this was not unconstitutional."

So about eight weeks ago the hospital began to offer its IVF patients the option of freezing the surplus zygotes. "The majority of the patients who were offered the option have been delighted to take it up," said Dr McKenna. He is confident that the practice lies within the Medical Council guidelines on infertility treatment, which specify that all fertilised eggs must be implanted, but do not specify when. "The wording of the guidelines might have been written with this in mind for the future," he said.

But what if the woman dies before implantation of the frozen second lot can take place?

It is the policy of the infertility programme at the Rotunda to transfer all frozen zygotes to the wife, he said, pointing to the consent form signed by all couples who participate in the programme.

This includes the following statements:

"It has been explained to us that it is the policy of HARI (Human Assisted Reproduction Ireland) that all Zygotes would be transferred to the wife by the end of her forty-fifth year of age or within five years of the freezing of the Zygotes, whichever is the earlier and we accept that all Cryopreserved or Stored Zygotes should be transferred to the wife within these periods. The wife was born on the . . . day of . . . 19 . .

"We consent to the transfer of all Cryopreserved and Stored Zygotes to the wife at a time to be determined in consultation with HARI . . . "

"In the event of either the death of the husband or the wife or both of us or in the event of a change of marital status of the husband or the wife we hereby agree that no transfer of Zygotes shall occur thereafter and that any and all Cryopreserved and Stored Zygotes shall be removed from storage and thawed without transfer by HARI and its successors and we consent to this."

The form also agrees that any frozen zygotes which have not been transferred to the wife within the specified time will be removed from storage and "thawed without transfer". It also warns the couple that the zygotes may not be suitable for freezing, or may not survive the procedure. Dr McKenna says that this consent form means that if the husband dies, any frozen zygotes will not be replaced in the widow, or if she dies they will not be replaced in any new partner he might have. Nor will the hospital implant the zygotes if the couple separates. "They are not the property of either partner but of both," he stresses.

Thus the hospital will avoid a situation where there might be a dispute over the ownership of the zygotes, or where a woman might seek to have a child without the consent of her former husband, with implications for his liability to provide support for that child.

The FACT remains that the couple agrees that the zygotes will be destroyed if one or both of them dies or if they separate, which implies that this course of action has been considered and agreed in principle by the hospital. Surely this runs counter to the Medical Council guidelines on the implantation of all fertilised eggs?

"It is clearly our intention to comply with the guidelines," said Dr McKenna. "We can't drag the woman in and tie her down and replace them. We will do all we humanly can to replace them. We believe they belong in the mother, not in experimental test tubes or in freezers. That is where they should be."

However, the chairman of the Ethics Committee of the Medical Council, Dr Jim Clinch, felt that allowing any of the zygotes to die was "not consistent with preserving life".

"The general view of the profession in Ireland is that doctors are there to preserve life and promote health," he said. "If a doctor feels an embryo is life he or she would do all in their power to preserve that life."

Eventually, especially if a complaint is made against the Rotunda, the freezing of zygotes might be tested in the courts. But the courts will face a number of problems, the main one being that nowhere in the Constitution is the term "unborn" defined.

This problem has already been highlighted by the Commission on the Constitution, which pointed to the curious use of the word, which is an adjective, but in the Constitution is used as a noun.

In the debate surrounding the amendment, the "unborn" was defined in relation to the "born", and was clearly understood to refer to the unborn baby in the mother's womb. But for lawyers dealing with this case the issue will be that the zygote is never in the mother's womb, so can it be "an unborn" and therefore fall within the terms of the constitutional amendment?