One embryo or two? A woman receiving IVF therapy can boost their chances of conceiving if two are transferred to her womb. But she may then also be more likely to have twins 鈥� and such children are at more risk of health problems. A new model could help solve the dilemma by weighing each woman鈥檚 probability of having twins against the likelihood of conceiving through IVF.
鈥淭wins are at four times the risk of early delivery, low birth weight and cerebral palsy,鈥� says Jan Holte at the Carl von Linn茅 IVF clinic in Uppsala, Sweden. The most obvious way to limit twinning in IVF is to transfer just one embryo. However, this is not a good solution for women in their late 30s and 40s, or other less fertile women, because it is too likely to leave them without even a single child.
Such women make up around a quarter of IVF cases, so the decision as to whether to transfer one embryo or two is important 鈥� yet it tends to be treated fairly arbitrarily, says Holte. 鈥淧eople don鈥檛 really know when to transfer two,鈥� he says. 鈥淭hey usually do if the woman is of an advanced age or if the embryo doesn鈥檛 look good.鈥�
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But, as far as embryos go, beauty is often in the eye of the beholder, Holte says. 鈥淚t鈥檚 a really subjective process.鈥� To overcome this initial hurdle, Holte and his colleagues developed a standardised embryo scoring system in 2006 ().
Picking winners
His team has been using the tool to investigate how much embryo quality, among around 80 other factors they measured, might influence a woman鈥檚 chance of having twins. Over four years, the group studied around 3000 embryos before transferring each to a woman and tracking her pregnancy.
鈥淲e found that there were four key variables 鈥� the woman鈥檚 age, her ovarian response to stimulation, her history of past treatments and pregnancies 鈥� and the embryo鈥檚 score,鈥� says Holte. The statisticians in his team then set about creating a model to predict pregnancy outcome based on these factors. 鈥淯sing our software, we can calculate in each case what the expected risk of twinning will be,鈥� says Holte.
Holte has programmed the software to recommend double embryo transfer only when the woman鈥檚 probability of having twins is below 15 per cent. Doing so helped reduce the total rate of twin deliveries at the clinic in Uppsala from over 26 per cent to 1.9 per cent 鈥� the same as the rate of twinning seen in naturally conceived births.
Tony Rutherford, chair of the , is impressed by the outcome. 鈥淭his model shows you can get [the twinning rate] down to normal, taking away the risks of multiple pregnancy associated with assisted reproductive technology.鈥�
鈥淲e see an enormous improvement in perinatal outcomes,鈥� says Holte. 鈥淔ar fewer babies are born prematurely and with a low birth weight.鈥� The software has already been adopted by other clinics in Sweden and Italy.
Holte will present the findings at the in Stockholm tomorrow.
