Successful pregnancy after transfer of human blastocysts that had naturally escaped completely from the zona on day 5

A viable human blastocyst must demonstrate the capacity to expand in vitro and hatch out from its protective zona pellucida after transfer, in order to implant into the endometrial lining. Some blastocysts, however, experience hatching difficulties and remain enclosed within their zonae, preventing implantation after transfer to the uterus. Successful pregnancies have been reported by transferring embryos that have been treated to partially remove the zonae by chemical intervention (e.g. pronase or acid Tyrode's) or mechanical intervention (e.g. by laser or use of assisted hatching pipettes). Complete zonae removal has also been reported prior to embryo transfer. This is a report of a case where expanded blastocysts naturally hatched in vitro and completely escaped from their zonae on day 5 of preimplantation development. The careful transfer of these fully expanded hatched blastocysts on the same day resulted in a successful clinical pregnancy.