Embryo vitrification’safe but longer storage cuts chances of pregnancy success’
Freezing and storing embryos during fertility treatment using a technique called vitrification is safe – but longer storage reduces chances of pregnancy success, research suggests.
Women are also less likely to have a live birth the longer the embryos are stored, the new study suggests.
There have been concerns that vitrification technology could be unsafe for the embryo, leading to complications at the time of birth and later.
Vitrification is the process of freezing the egg so rapidly that the water molecules don’t have time to form ice crystals, and instead instantaneously solidify into a glass-like structure.
When needed, the freezing process is reversed to thaw and warm the embryos quickly.
Co-author Professor Qifeng Lyu is deputy director of the department of assisted reproduction at Shanghai Ninth People’s Hospital, China, and said: “Our study suggests that although the storage time of vitrified embryos negatively affected pregnancy outcomes, including clinical pregnancy and live birth rates, it did not affect neonatal outcomes.
“Our study demonstrated the safety of using long-stored embryos after vitrification on neonatal health.
“This is reassuring news for couples seeking fertility treatment.”
The researchers analysed outcomes from 24,698 patients who had vitrified embryos transferred for the first time between January 2011 and December 2017.
They compared patients who had vitrified embryos stored for up to three months -group one, with patients whose embryos were stored for three to six months – group two, six to 12 months – group three, and 12-24 months – group four.
Researchers found that implantation rate fell from 40% in group one to 26% in group four.
The clinical pregnancy rate fell from 56% in group one to 26% in group four, and the live birth rate fell from 47% in group one to 26% in group four.
The scientists say this means that among women who had embryos stored for less than three months, 47 out every 100 women would have a live birth.
But among women who had embryos stored for between 12-24 months, 34 out of every 100 women would have a live birth.
The rate of miscarriages and ectopic pregnancies also increased with longer storage time, according to the study published in Human Reproduction.
However, these associations were not statistically significant after taking into account factors that could affect the results such as the mother’s age, mother’s body mass index, the cause of infertility, parity and embryo quality and stage of development.
There was no evidence that storage time affected neonatal outcomes.
Groups three and four had a greater proportion of older women or patients with a poor prognosis due to smaller numbers of available eggs than groups one and two.
So the researchers analysed a subset of 7,270 women who were younger than 36 years and whose infertility was caused by blocked or damaged fallopian tubes.
This produced similar results with a 50% live birth rate among women in group one, compared to a 38% live birth rate in group four.
The study had a number of limitations, including that it did not to show the effect of storing embryos for longer than 24 months, and the researchers did not undertake long-term follow-up of babies, so do not have information about their growth and development.
Dr Qianqian Zhu, a research assistant who led the study, said: “Our results suggest that clinicians should consider the effect of storage duration before making decisions about the numbers of embryos to freeze and store.”
Dr Gill Lockwood, medical director at Care Fertility Tamworth, said: “This is an important study because it includes 25,000 patients, but it suffers from being both retrospective and covering a period of six and a half years during which much progress has been made worldwide with vitrification techniques.”
Dr Meenakshi Choudhary, consultant gynaecologist and sub specialist in reproductive medicine, Newcastle upon Tyne Hospitals NHS Trust, and Newcastle Fertility Centre at Life, said: “In the UK and in many countries across the globe, clinics are moving toward blastocyst transfer.
“However in this retrospective study, over 90% embryo transfers done were of cleavage stage embryos when the embryo is up to 3 days old.
“One needs to exercise caution to extrapolate this for blastocyst stage outcomes as only 54 blastocyst transfers took place in women with extended storage of 12-24 months.”