Dr Jan Holte told the annual meeting of the European Society for Human Reproduction and Embryology, that if this model were to be applied in all fertility clinics, it had the potential to reduce the twin rates to the level of just under two percent seen in the normal population. Already, another four clinics have started to use the model.
Sweden leads the world in attempts to reduce multiple pregnancies by transferring only one embryo to a woman’s womb during fertility treatment whenever possible. In 2007 , 69.9% of embryo transfers were single embryos, 30.1% were double embryo transfers, and only 5.3% of deliveries after fertility treatment were multiple deliveries – the lowest multiple delivery rate in the world.
“However, until now, evidence-based strategies have been lacking for guidance on when to perform single embryo transfer in order to achieve the best possible balance between maintaining a high level of live birth rates, but reducing twin implantation rates,” said Dr Holte, who is medical director, a senior consultant and responsible for research at the Carl Von Linnéklinikken, Uppsala Science Park .
Over a four-year period between 1999-2002 Dr Holte and his colleagues analysed a series of 3223 embryo transfers and recorded 80 different factors that played a role in the success of fertility treatment. They found that four variables were significant in predicting pregnancy outcome: the quality of the embryo, the age of the woman, ovarian responsiveness , and information about whether the woman had had previous IVF attempts with either fresh or frozen-thawed embryos, how many, and whether or not they resulted in a pregnancy.
Using these four variables, they constructed a mathematical model that predicted the chances of pregnancy after the transfer of one or two embryos and of the risk of twins. Over a subsequent four-year period between 2004-2007, they applied the model in the clinic for 3410 embryo transfers. They aimed to achieve a risk of twins of no more than 15% and any women that had a higher risk had only one embryo transferred in one cycle. Transfers of embryos that had been frozen, stored and then thawed before implantation followed the same model.
During this period the proportion of single embryo transfers increased to 76.2% , and the rate of twin deliveries was reduced from 26.1% to 1.9%. Live birth rates per fresh embryo transfer fell from 29.1% to 24.6%, but when transfers of frozen-thawed embryos were included the live birth rate was similar during the two periods: 31.1% in the earlier period and 30.7% in the later period.
Dr Holte also made adjustments to take account of the fact that women in the later period tended to be older and have a less favourable prognosis than those in the earlier period and, therefore, would have a lower predicted live birth rate. Once he had done this, the live birth rate in the later period was higher at 36% versus 31.1% in the earlier period.
Just as significant as the dramatic drop in twin births were the outcomes for babies born between 2004-2007. Average birth weights increased from 3086g to 3412g; the frequencies of babies born prematurely and babies with birth weights below 2500g were reduced by two-thirds; the frequency of babies born small for gestational age was reduced by 26%; and deaths either just before, during or just after birth were reduced by 58%.
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