A blastocyst describes an embryo that has developed for approximately 5 days; the stage at which an embryo would naturally arrive into the uterine cavity. Blastocyst culture offers most benefit when there are several embryos to choose from as it allows the embryologist to select strong embryos that are more likely to implant. If there are fewer numbers, then your embryologist might recommend transferring them at an earlier stage, as not all fertilized eggs reach the blastocyst stage of development. To reduce the risk of multiple pregnancy, remaining blastocysts can be frozen for use at a later date.
Blastocyst culture especially suits couples where there is a compelling medical reason that would make a twin or triplet pregnancy very dangerous for both mother and baby. It is also recommended for women aged 36 and under who have a high risk of having twins. Twinning can still occur even if just one embryo is replaced. In this instance the embryo has divided. This is known as monozygotic twining and can occur in 2-3% of cases.
The success of blastocyst culture depends simply on the number and quality of embryos available. One of the main risks is not having a transfer, as its possible that the embryo can arrest in development from day 3 onwards. This is the reason we only offer the service to certain couples. It's important to remember too, that it is likely there will be fewer embryos available to freeze as some may have arrested in their development.