The University Hospitals Fertility Clinic in Cleveland said more than 4,000 eggs and embryos were lost in the recent tank failure, according to an article on the CNN website.
About 950 families were affected by March incident.
"The technical manner in which the eggs and embryos are stored in these freezers complicated our initial determination of how many patients and specimens were affected," a letter from the facility stated.
At the time of the failure, the clinic said it was unable to determine right away whether any of the eggs and embryos were still viable. In the letter this week, it said, "We are heartbroken to tell you that it's unlikely any are viable."
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