There are many pros and cons associated with both single and multiple embryo transfer. The best number of embryos to transfer will depend on your age and the quality of your embryos.
Although the UK law allows for up to two embryos to be transferred in women under the age of 40, it is now common practice to transfer only one embryo and freeze any remaining good quality embryos. This is because single embryo transfer reduces the risk of multiple pregnancies (twins, triplets or more), which can be associated with health risks to both the mother and the babies. Health risks of multiple pregnancy include increased miscarriage risk, premature birth and low birth weight. Research suggests that single embryo transfer is safer and as effective as multiple embryo transfer.
For women above the age of 40, the law states that up to a maximum of 3 embryos can be transferred. This is because success rates decrease with age, so older women have a reduced chance of successful embryo implantation when only one embryo is transferred.