The risk of birth defects increases for the surviving baby when its twin dies early in the mother's womb, Associate Professor Michael Davies of the University of Adelaide has said. He is the co-director of the Research Centre for the Early Origins of Health and Disease at the university.
The "vanishing twin" phenomenon could perhaps be linked to a nearly two-fold increased risk of congenital malformation in the surviving baby, and a threefold increase in multiple birth defects.
The phenomenon occurs when there are fewer babies born than detected in early pregnancy by ultrasound - often reducing from twins to single babies but also possibly from triplets to twins, Associate Professor Davies explained at the annual meeting of the European Society of Human Reproduction and Embryology in Stockholm.
He says the discovery will now pave the way for researchers to investigate factors that influence embryo development and loss.
"This has significant potential for advancing our understanding of the origins of congenital malformation, not just after infertility treatment, but also in natural pregnancies," Associate Professor Davies says.
"It is difficult to study what factors in early pregnancy might cause congenital malformation, such as heart and skeletal defects and cerebral palsy. This is because, in the general population, the majority of miscarriages - including vanishing twins - occur in the early days and weeks of pregnancy, often before the woman is even aware she is pregnant.
"However, in women undergoing fertility treatment, early pregnancy is much easier to study because doctors know exactly when eggs were fertilised and transferred to the woman's womb. The first ultrasound scans are usually carried out around six to eight weeks. This is followed by close monitoring with pregnancy tests and ultrasounds from the very beginning."
Associate Professor Davies and his University of Adelaide colleagues studied data from all assisted reproductive technology (ART) cycles that took place in South Australia between January 1986 and December 2002, linking them to registry data on birth defects and cerebral palsy.
They identified cases in which a foetus had been lost by comparing routine six-eight week ultrasound data - which shows the presence of an empty foetal sac - and the number of babies actually delivered. These results were compared with single, successful pregnancies.
A total of 7462 IVF babies were delivered in South Australia in this 16-year period. In those multiple pregnancies where only one twin survived, 14.6 per cent of babies born had congenital malformations. The presence of an empty foetal sac at six weeks nearly doubled the risk of a malformation and nearly trebled the risk of multiple malformations.
Associate Professor Davies also looked at pregnancy loss after the first six-eight week ultrasound and found a link with birth defects in the surviving twin as well, but not as important as early loss.
"To our knowledge, this appears to be the first association of very early loss of a co-twin and a range of congenital malformations. It demonstrates that the quality of embryos in IVF twins is related. Where one fails to develop, it appears to be an important indicator of the health of the survivor."
Associate Professor Davies says the results show without doubt that the quality of embryos is paramount in a successful pregnancy.
"This will have important implications for fertility treatment. Twinning continues to be a high-risk pregnancy for mother and baby in any situation, but creating and using multiple embryos of lower quality may also increase the risk of both miscarriage and birth defects.
"This breakthrough should help us pinpoint the factors relating to embryo quality and therefore drastically reduce the risk of birth defects in IVF babies. It's plausible that these same factors will operate in natural pregnancies," he says.