The belief that IVF is behind the steady increase in the numbers of twins born over the past thirty years was challenged yesterday by a scientist speaking at the European Society of Human Reproduction and Embryology conference. Professor Robert Jansen, Medical Director of Sydney IVF in Australia, said that his research had shown that improved nutrition, both maternal and in the lab (in the case of IVF), was producing better and stronger embryos.
"Over the last 100 years there has been an increase in identical twinning through the division of the embryo into two, even without IVF", said Jansen, "and with the move to single embryo transfer with IVF this trend is obviously set to continue." He added 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.
Jansen's research used statistics from 1920 to 2003 to determine the sex of babies at birth among multiple pregnancies. He 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 a 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 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 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 minimizing MZ as well as DZ twinning is to transfer one embryo at a time, irrespective of the age of the mother to be", he added.
Source: European Society for Human Reproduction and Embryology