IVF cycles often produce more than one embryo. As a result, there may be unused embryos after embryo transfer. If these embryos are of suitable quality, they can be frozen (or cryopreserved). In some cases, for instance, when the risk of OHSS is high, all embryos from the cycle will be frozen to prevent OHSS from developing. Embryo freezing allows you to have more than one pregnancy from a single IVF stimulation cycle.
Frozen embryos are thawed and replaced in Frozen Embryo transfer (FET) cycles. Following thaw, more than 90% of embryos return to a very similar state to what they were before being frozen. A small proportion of embryos do not survive thawing. FET cycles can be completely natural in women who ovulate regularly. In natural FET cycles, the body’s own hormone production is used to prepare the lining of the womb to receive the embryo and no additional medication is needed. Alternatively, FET cycles can be medicated, and involve the use of oestrogen and progesterone medications to prepare the lining.