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The impact of duration of freezing of IVF embryos on pregnancy and perinatal outcomes – analysis of UK national data

Chief investigator: Edwin Amalraj Raja
Research establishment: University of Aberdeen
Year of approval: 2023

Lay summary

IVF (in-vitro fertilisation) is the recommended treatment for couples with unresolved infertility. It involves mixing eggs and sperm to create embryos which are then cultured in the laboratory. Usually, the best embryo is then put back in the womb, while others can be frozen for future use. The use of frozen embryos has become popular across the globe in recent years, accounting for 41% of all IVF treatments in 2019 in UK. Although many embryos are frozen for short durations, others can be kept in the freezer for many months or years and it is believed that patients can use them at any time without affecting their chances of pregnancy rates or the health of their babies. This study will use information of treatments undertaken between 2000 and 2019 from the Human Fertilisation Embryology Authority (HFEA) database which holds data of all IVF treatments undertaken in the UK licensed clinics, to explore the impact of the duration of embryo freezing on IVF live birth rate and safety of children. The main outcome measures are live birth, gestational age (preterm vs term), birthweight (small/large for gestational age vs normal for gestational age) and congenital anomalies. The results of the proposed research will provide patients and clinicians with the necessary information so that they can make informed choices in the future.

Public benefit statement

While the general view is that prolonged embryo storage is safe, there has been very little research on this subject and the published evidence is conflicting. While many women at risk of producing fewer eggs are increasingly favouring the use of frozen banked embryos for treatment, there is little research on whether this increases their chances of success. Thus, this research is necessary, timely and relevant to people in the U.K. A national study involving data from several hundreds and thousands of women will provide robust findings necessary to counsel patients and inform joint decision-making around the appropriate duration of freezing embryos to ensure maximum chances of IVF success and minimise risks to the baby. In addition to live birth rates, the impact of different freezing durations could influence the chances of preterm birth, birth weight and congenital abnormalities. More information on these outcomes will help inform millions of fertility patients across the UK and beyond to make informed decisions.

Review date: 2 May 2026