Many scientific studies have addressed this numbers question. The conclusion: more eggs mean the likelihood of more embryos for transfer or storage. Also, a better chance of having older embryos (blastocysts) for transfer.
Such views were underlined by a well quoted British study in 2011 that reviewed more than 400,000 IVF cycles in the HFEA database. It found that the best chance of live birth was seen when around 15 eggs were collected. However, as many other studies have shown, the chance of success began to decline after 20 eggs were retrieved.
What the studies say
A new study from the USA has also found that the chances of pregnancy and live birth do increase significantly with every extra fertilised egg available. But, only up to a total of nine and decline after that.
However, it's important to note that these results were calculated from more than 15,000 treatments in a first fresh cycle - and only with a single blastocyst transfer.
The delivery rates were not calculated cumulatively. Fresh plus subsequent frozen cycles, which tend to be a more realistic measure of success today. The calculation was also based on fertilised and not freshly collected eggs.
“more eggs mean the likelihood of more embryos for transfer or storage. Also, a better chance of having older embryos (blastocysts) for transfer”
A study from 2018 checked the records of almost 15,000 patients in several clinics in Europe. They found cumulative outcomes continuing to improve with an ever-increasing number of eggs available. The outcomes reached a delivery rate of 70% when more than 25 eggs were retrieved at the first egg collection cycle.
In this study, as in most others, no plateau effect was observed. Although the cumulative rate did fall when initial cycles collected 27 oocytes.
But all these studies were in conventional IVF and ICSI. What about the best chances of live birth in banked egg donation treatments or elective egg freezing cases?
Our best evidence here comes from Spain. Specialists investigating egg donation treatments found that the viability of eggs fast-frozen by vitrification were just as viable in terms of outcome as eggs collected fresh from donors for fertilisation and transfers.
It was these studies in Spain which opened the door for frozen egg banking in egg donation and established egg freezing as an efficient option for egg donation and fertility preservation. And the same group too who in a study of 1400 women having egg freezing for various reasons also found that the probability of having a baby increased progressively according to the number of eggs used. The increase in probability was found to be rapid until egg numbers reached 10-12, and slower from the 20th onwards.
However, a more recent study has also shown that the age at which the eggs are frozen is an important factor. This study was based on the outcomes of 520 egg freezing patients in the USA. They found that a 35-year-old woman with ten eggs had a 69% chance of a baby. Two years later, at age 37, that chance had reduced to 50% - and so on such that at the age of 39, she had a 39% chance.
What's the consensus?
The data on which to answer patient questions about the precise number of eggs necessary for the best chance of a baby is not clearly available.
Egg freezing for personal reasons is still a relatively new procedure. There are simply insufficient numbers to give any scientifically accurate answer. But what will certainly be important is the age of the woman who freezes her eggs, whether egg donor or elective egg freezer.
Currently, according to the studies, the best chances seem probably in those under the age of 38 and have between 10 and 15 eggs available. It's important to remember, however, that the evidence is only slowly accumulating, and it's not always consistent.