The paper published in Fertility and Sterility investigated the success of the techniques in nearly 83,000 first-time IVF patients between 2014 and 2015. Around 83 percent of patients had a fresh transfer for their first IVF cycle versus 17 percent who had a frozen transfer. The study found that the number of eggs a patient produced at collection was linked to differing success rates of fresh and frozen embryo transfer.

‘The bottom line for patients is that they’re likely seeing a lot of information out there saying frozen transfer is best all of the time, and we are seeing that may not be the case,’ said study author Dr Kelly Acharya, an endocrinologist at Duke University in Durham, North Carolina.

Frozen embryo transfers had added benefits only for patients who produced large numbers of eggs – 15 or more – at collection. These patients had slightly higher birth rates for frozen embryos, at 52 percent, compared with fresh embryos, at 48 percent. However, patients who produced low (one to five) or intermediate (six to 14) numbers of eggs had higher birth rates with a fresh embryo than a frozen embryo.

There is much debate in the field about whether fresh or frozen embryo transfer produces has the best clinical outcomes. Elective freeze-all embryo cycles are growing in popularity as new evidence supporting their success increases. This study overturns the idea that a freeze-all strategy is best for all IVF patients.

‘In the past five years, some clinics have advocated for freezing everything with the belief that the process that stimulated the eggs makes the lining of the uterus less welcoming for pregnancy,’ said Dr Suheil Muasher, also an author of the paper and a reproductive endocrinology and infertility specialist at Duke University. Some previous evidence suggests that pregnancies from a frozen embryo transfer are less likely to result in a premature birth or underweight babies, he added.

Dr Acharya noted that there are also advantages in using fresh embryos, as there are fewer cycles of hormones and less waiting involved between egg collection and implantation. ‘Freezing also can lead to another one or two months of waiting and not knowing whether the procedure will be successful, which can be emotionally draining for patients,’ she said.

The authors concluded that clinics should avoid blanket ‘frozen is best’ policies. Instead they should factor in patients’ egg numbers when recommending frozen or fresh embryo transfers to boost chances of a successful pregnancy and birth.