Women who have only one embryo transferred during IVF are almost five times more likely to deliver a baby at full term than those having two embryos put back, according to new research.
Babies are also far more likely to be born at healthy weights if they are the result of a single rather than multiple pregnancy, the review of existing studies found.
Fertility clinics across the UK are coming under increasing pressure to reduce the number of multiple pregnancies resulting from IVF.
Critics argue that the poor provision of IVF on the NHS – despite guidance saying women should get three free cycles – means some couples feel desperate to have two embryos implanted.
Twin and triplet pregnancies are linked to higher chances of premature deliveries, low birth weights and miscarriages.
These risks have not consistently been shown to be higher for IVF twins than those conceived naturally.
The study, published in the British Medical Journal, involved 683 women having single embryo transfers and 684 having double transfers.
Overall, the live birth rate using single fresh embryos was lower than when two fresh embryos were transferred.
The authors, led by a team from Aberdeen University, concluded: "Elective single embryo transfer results in a higher chance of delivering a term singleton live birth compared with double-embryo transfer."
Some NHS trusts are cutting IVF altogether to save costs.