Vitrification of human embryos previously cryostored by either slow freezing or vitrification results in high pregnancy rates

Occasionally, clinical scenarios arise where embryos, previously cryostored and warmed, need to be recryopreserved. The outcome of 30 such transfer cycles from 25 women where embryos were recryopreserved is detailed. In 16 cases, embryos were initially cryopreserved by slow freezing and in 14 cases by vitrification. The cryopreservation stages were the pronuclear stage (n = 16), day-3 cleavage stage (n = 12), blastocyst (n = 1) and oocytes (n = 1). All recryopreservation was by Cryotop-based vitrification. From this mixed source, 30/31 twice-cryopreserved embryos survived warming and were transferred, resulting in 13 pregnancies, 11 deliveries with normal gestational age and birthweight, one pre-term birth at 33 weeks and two miscarriages. There were no malformations reported for the live births. Recryopreservation using vitrification by CryoTop has been used in a variety of clinical scenarios to preserve surplus cryopreserved embryos. The current study, although limited in numbers, resulted in high survival rates, clinical pregnancy rates similar to once-cryopreserved embryos and healthy live births independently of the initial stage and cryopreservation method. The technique may increasingly be applicable to elective single-embryo transfer and blastocyst transfer to maximize the pregnancy rate while minimizing the number of cryopreserved embryo transfers.Freezing of embryos has significantly influenced the likelihood of pregnancy after an IVF stimulation cycle. Since it is often unclear which embryos have the potential to continue developing, more embryos may be thawed than transferred. If, in such cases, the surplus embryos remain viable, the ability to refreeze them is an important laboratory tool. In this study, we report on 30 cycles from 25 women where, for a variety of clinical scenarios, surplus embryos previously cryopreserved by either slow freezing or more recently by vitrification were recryopreserved using the Cryotop vitrification protocol. The report details 13 pregnancies, including 11 live births. The study group included one case where oocytes rather than embryos were initially vitrified. When these recryopreserved embryos were subsequently warmed, they exhibited a high survival rate and, after embryo transfer, a pregnancy rate similar to once-cryopreserved embryos. While recryopreservation has to date demonstrated that the embryos retain viability, leading to healthy children, and is, as such, an additional tool for embryologists, its application should proceed with caution.