He said there was an "almost zero chance" of Ms Allwood successfully giving birth to the babies, conceived with the aid of fertility drugs, and he appealed for her to be left in peace to decide her future away from the influence of the media.
Ms Allwood, 31, and her partner Paul Hudson, 37, from Solihull in the West Midlands, have reportedly sold their story to the paper for pounds 1m, or pounds 125,000 per baby, amid suggestions there was a sliding scale according to the number of embryos which survived.
Professor Kypros Nicolaides, head of foetal medicine at King's College Hospital, London, said: "It is absolutely imperative that within the next two or three days the publicity settles to allow her, her partner and medical team to make the right decision." The safest option would be to reduce the embryos to two, he said.
The News of the World has denied operating the worst kind of "cheque- book journalism" and said that the contract with Ms Allwood was not a "step-by-step" arrangement. In a statement it also said that if she chose to withdraw, the newspaper "would not for a moment stand in her way".
Stuart Kuttner, managing editor of the newspaper, said: "The last thing the News of the World would want would be for Mandy Allwood to take any risk to herself or to the children. We are not in that kind of business."
According to Max Clifford, the PR guru who is acting for Ms Allwood and Mr Hudson, the couple are giving priority to her health and that of the embryos, at a private address where they are in hiding, accompanied by a News of the World reporter.
Mr Clifford said: "They received money for the exclusive story, which they will get whatever happens. With regards to the future, it's got to be very flexible. She's hoping for eight healthy children."
He added: "She's also been told if she were to abort, and she doesn't believe in abortion, it might then kill the other children. But if it might also a question of aborting two because that was the only chance of saving the other children, it's not a question of having eight children at all costs."
Leading fertility specialists yesterday resisted calls for greater regulation of routine fertility treatments of the type Mandy Allwood underwent.
The failure in Ms Allwood's case, they argued, was not one of regulation but of her apparent failure to take medical advice not to have sexual intercourse at a criticial stage in her treatment. Some specialists were sceptical of her claims that she underwent the treatment behind Mr Hudson's back and queried why she was being treated for infertility at all.
Mr Robert Forman, clinical director of the London Gynaecology and Fertility Centre, said Ms Allwood's treatment was a routine one which had been available for the past 30 years. It had produced the Walton sextuplets in 1983, and multiple pregnancies were a recognised risk, with about one in four successful treatments producing twins.
Very high numbers of conceptions, however, were now "exceedingly rare" because patients took their doctors' advice about avoiding sexual intercourse if a large number of eggs were produced.
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