"Over the last 100 years, both in the UK and Australia, there has been an increase in identical twinning through the division of the embryo into two, even without IVF", he said, "and with the move to single embryo transfer with IVF this trend is obviously set to continue." Professor Jansen went on to say that the present rate of identical twinning with IVF is between one-in-a-hundred and one-in-fifty, a little over twice the rate involved when getting pregnant naturally.
Professor Jansen and his team reviewed Australian national birth statistics from 1920 to 2003 to determine the sex of babies at birth among multiple pregnancies. They found that the rate of dizygotic (DZ) twinning – where two embryos are involved and half the twins will be of different sex - was relatively constant from 1920 until the 1960s, but there was then the well-known dramatic increase with the advent of induced ovulation and IVF – reaching 300 in every 1000 IVF conceptions by 2000. Among monozygotic (MZ) twins, caused by embryo division (so all are of like sex), the excess rate of same-sex twins among natural conceptions has risen steadily for the last 80 years. MZ twins were relatively rare among IVF babies in the 1980s – much less than occurs naturally – but then rose in the nineties to reach 14 per 1000 by the year 2000.
"As IVF techniques improved there was a steady and substantial increase in MZ twinning, but starting from a low base – much lower than with natural conceptions", said Professor Jansen. "Our study shows for the first time that the increase we've seen coincides with improving culture conditions and that it started before any manipulation of the zona – the 'shell' of the egg – began. Transfer of just one embryo can therefore still lead to twins. If a higher rate of MZ twinning turns out to be a natural adaptation to improved nutrition or culture conditions, it could prove difficult to reverse as embryo quality continues to improve – whether in the IVF lab or in the community generally."
"Because two implanted embryos have twice the chance of MZ twins than a single implanting embryo the best way of minimising MZ as well as DZ twinning is to transfer one embryo at a time, irrespective of the age of the mother to be", he said.
In another other presentation to the conference, Mr. Graham Scotland, from the Health Economics Research Unit, University of Aberdeen, Aberdeen, UK, described how he and his colleagues had conducted work that throws light on why some couples prefer double embryo transfer (DET), despite being aware of the risks of twin pregnancy. His team assessed patients' preferences for a range of adverse birth outcomes associated with twin pregnancies – cerebral palsy, cognitive impairments, visual impairments, perinatal death – as well as a scenario in which treatment failed and they did not become pregnant. Participants were asked to read several cards describing the treatment outcomes and then asked hypothetical questions to elicit their preference value for each.
"The results showed that many women awaiting IVF appear to prefer the prospect of a chronic adverse birth outcome rather than childlessness", said Mr. Scotland. "Having no child at all was valued significantly lower than having a child with physical, cognitive or visual impairments, but significantly higher than perinatal death. Our work suggests that couples may need to be convinced that their chances of live birth with single embryo transfer are as high as they would be with DET, before they will voluntarily accept it."