Blastocyst transfers increase the likelihood of pregnancy, but there is an increased risk of identical twins. If more than one blastocyst is transferred there is the possibility of triplets, where both blastocysts implant and one of those blastocysts splits. Identical twins have an increased chance of obstetrical complications compared to non-identical twins. However, blastocyst transfer also allows the option of single embryo transfer.
A single embryo transfer minimizes the chance of twins, which is particularly important when twins would be more risky, such as with a misshapen uterus due to certain uterine abnormalities. Any couple wishing to avoid the chance of twins for any reason may choose single blastocyst transfer. Another advantage of developing all embryos to the blastocyst stage is that it makes embryo cryopreservation (freezing) more efficient, since embryos without the capacity for full development are not frozen.