"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 bas
Source:European Society for Human Reproduction and Embryology