The vitrification method of freezing and storing embryos is safe, but the length of time embryos are kept in storage may affect their chances of resulting in a pregnancy and successful birth.
Researchers based in the Department of Assisted Reproduction at Shanghai Ninth People's Hospital, China, looked at the outcomes of over 24,000 women who had vitrified embryos transferred. They found that although longer storage was associated with decreased pregnancy and live birth rates, it did not affect the health of newborns.
'This is reassuring news for couples seeking fertility treatment', since 'the reduction in live birth rates can be overcome through additional embryo transfer cycles', noted co-author Professor Qifeng Lyu.
The study, published in Human Reproduction, divided women into four groups based on their length of embryo storage time. They found that implantation rate fell from 40 percent to 26 percent, and that clinical pregnancy dropped from 56 percent to 26 percent when comparing embryos stored for up to three months with those kept for between 12 and 24 months. Based on their findings, they suggest that clinicians should consider the effects of storage duration before deciding how many embryos to freeze and store.
'Fertility patients who have embryos in storage should not panic', said Sarah Norcross, director of fertility and genomics charity the Progress Educational Trust. 'The data is retrospective, from a single centre and only covers a short period of time. It is unlikely that the length of storage of embryos is the only factor at play', she said, stressing that this research is not conclusive.
Fertility scientists are also apprehensive about the results, which, according to Dr Bassel Wattar, clinical lecturer in women's health at the University of Warwick – who was not involved in the research – are 'prone to selection bias'. For example, women who stored their embryos longer also tended to be older and have a poorer prognosis, which could alternatively explain the lower pregnancy and birth rates of these groups.
The researchers attempted to correct for these differences and reported similar findings even when they analysed in a subset of women under 36, whose infertility was caused by blocked or damaged fallopian tubes. Dr Qianqian Zhu, lead author on the study, claimed that this 'supports our main results about the relationship between the duration of storage with pregnancy and neonatal outcomes'.
However, Ying Cheong, professor of reproductive medicine at the University of Southampton, who was not involved in the study, pointed out that the data is not relevant to the freezing of 'blastocyst-stage embryos, which most IVF centres use'. In addition, the study did not examine effects beyond 24 months.