Vitrification of human blastocysts: an update.

Transfer of blastocyst-stage embryos has been shown to increase pregnancy rates while allowing for improved selection of potentially viable embryos. At this late stage of development, lower numbers of embryos can be transferred, resulting in less high-order multiple pregnancies and increased implantation rates. Between January 2004 and February 2009, 8449 blastocysts from 2453 patients were vitrified. After 1398 vitrified embryo transfers (VET) of both day-5 and day-6 blastocysts with a mean patient age of 34.6 +/- 5.0 years, the study centre has seen a survival rate of 96.3% (2730/2835), an implantation rate of 29.4% and a clinical pregnancy rate per VET of 42.8% (599 pregnancies/1398 warmed embryo transfers). After more than 5 years of vitrifying blastocysts, the perinatal outcome was, from 348 deliveries with vitrified blastocysts, the births of 431 babies (202 boys and 229 girls). One of the benefits of blastocyst vitrification is that it can be undertaken on a more flexible basis by laboratory staff. Also, vitrification may allow individual blastocysts to be cryopreserved at their optimal stage of development and expansion.