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Vitrification is a safe way to freeze and store embryos during fertility treatment, but the longer embryos are stored, the less likely women are to get pregnant and have a live birth, a new study from China suggests.
In vitrification, embryos are briefly placed in a dehydrating solution, then fast-frozen to prevent damaging ice crystals from forming.
Some experts feared the process could be unsafe for the embryo, leading to complications, including preterm birth, low or high birth weight and birth defects.
For this study, researchers in Shanghai analyzed data from nearly 24,700 women who had vitrified embryos transferred for the first time between January 2011 and December 2017.
Group one had embryos stored for up to three months; group two embryos had been stored for three to six months; group three embryos were stored six to 12 months; and embryos in group four had been stored one to two years.
The embryo implantation rate fell from 40% in group one to 26% in group four, while the clinical pregnancy rate fell from 56% in group one to 26% in group four. The live birth rate fell from 47% in group one to 26% in group four.
There was no evidence that storage time affected infant outcomes, according to the study published June 24 in the journal Human Reproduction.
Study co-author Qifeng Lyu is deputy director of the Department of Assisted Reproduction at Shanghai Ninth People's Hospital.
While the study suggests that storage time had a negative effect on implantation and live birth rates, Lyu said it demonstrated the safety of using long-stored vitrified embryos on newborn health.
"This is reassuring news for couples seeking fertility treatment. The reduction in live birth rates can be overcome through additional embryo transfer cycles. If we had found that neonatal health was adversely affected by vitrification, it would impose a heavy burden on individuals, family and society," Lyu said in a journal news release.
Study leader Qianqian Zhu, from the same hospital department, said clinicians should consider the effect of storage time before deciding how many embryos to freeze and store.
"This is especially important for cancer patients, who may have their ovaries destroyed by cancer therapies and who have to delay fertility treatment until they have recovered from their disease," Zhu said.
-- Robert Preidt
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