This is a procedure which is used to freeze either surplus embryos or to freeze all embryos of an IVF cycle These embryos can be cryopreserved by freezing them in the laboratory using specialized freezing equipment and the embryos can then be safely stored in liquid nitrogen. The method used in our lab is called vitrification. Through a series of carefully orchestrated steps, the embryos are ultimately frozen at temperature of minus 196 degrees below zero leaving them in a state of suspended animation in which they can remain for many years. Even embryos that have been stored for more than 5-6 years have successfully generated pregnancies (although most patients tend to use their frozen embryos within 3 to 5 years after they are frozen). We have successfully given pregnancy to a lady with enbryos frozen in our lab 9 years back.
(https://timesofindia.indiatimes.com/city/delhi/embryo-frozen-for-9-years-used-for-ivf-birth/articleshow/67169513.cms) The pregnancy rates associated with replacing frozen embryos depend on the age of the patient and the quality of the embryos at the time of cryopreservation.