The quality of human eggs fertilized in vitro and frozen is not affected by how long they are stored, as long as it is less than nine years, according to the results of a survey released Thursday by a hospital in Hokkaido.

Frozen fertilized eggs can be thawed and returned to the womb. Eggs fertilized in vitro and frozen can be stored for years. In one case in Israel, a healthy child was delivered from a fertilized egg that had been stored for 12 years. But questions remain about how safe it is to store frozen eggs for extended periods of time.

The results of the study, conducted by a team led by Hirofumi Hemmi, a specialist in reproductive endocrinology at Tonan Hospital in Sapporo, may reassure those who have eggs in storage.

“At least we know that the quality and other factors will not be affected for eight or nine years,” Hemmi said.

The research covered 1,790 women who underwent fertility treatment there between 1994 and 2005.

The pregnancy rate for frozen fertilized eggs was about 20 percent. While about 29 percent of the pregnancies ended in miscarriage, the rates were not affected by the storage times.

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