Objective: To report an ongoing twin pregnancy after transfer of embryos that were vitrified at the 2-pronuclei stage in a new vitrification solution.
Design: Case report.
Setting: A tertiary-care infertility clinic.
Patient(s): A 26-year-old infertile woman in whom two previous IVF implantations failed.
Intervention(s): Vitrification of 2-pronuclei embryos, in vitro culture for 48 hours, and transfer into the uterus.
Main outcome measure(s): Survival and cleavage after vitrification and achievement of clinical pregnancy.
Result(s): Six zygotes were vitrified by using a three-step protocol (4% ethylene glycol for 3 minutes, 20% ethylene glycol for 1 minute, and 38% ethylene glycol and 1.2 M trehalose for 0.5 minute). After 2 months of storage in a double-straw system in liquid nitrogen, two zygotes were warmed and cryoprotectants were removed by using a four-step protocol (1 M, 0.5 M, 0.25 M, and 0.125 M of trehalose). Two embryos were transferred after 48 hours of in vitro culture, cleaving to 5 and 6 cells. The resulting twin pregnancy was confirmed by ultrasonography at the sixth week.
Conclusion(s): Vitrification by using ethylene glycol and trehalose appears to be a safe, promising method for cryopreservation of human zygotes. Storage of vitrified zygotes in a double-straw system does not compromise their subsequent potential for survival and development.