How many eggs should we thaw at a time?
Dear Robert,

My partner and I are currently undergoing a frozen egg donation cycle. We successfully fertilised 8 eggs but are now nearing the end of that supply and wanted to ask your advice.

On our first attempt we were advised to thaw 4 eggs, and the two that survived the thaw on day 2 were transferred. On our second attempt we thawed 2 eggs (of medium quality) and were advised to transfer them on day 2 again. Both of these attempts were unsuccessful.

We now have 2 eggs remaining and are considering our options. In the are where we live, this is our only free attempt at IVF. Within that attempt we can have 4 transfers.
The clinic suggested it is better to have 2 eggs implanted but we read a recent small-scale study suggesting implanting 1 egg could be better. Therefore are currently unsure how to proceed.

We wondered if you would advise thawing one egg at a time? If so, do you recommend we grow the egg on further (e.g. to 5 days) or transfer straight away? The evidence base is difficult to decipher and apply to our situation, so would really appreciate your input.

My partner is considering taking baby aspirin as we read this can be helpful with implantation in egg donation. Again, would you advise this or anything else? For example, we are considering whether to have a test for Natural Killer Cells as my partner has endometriosis, and we have read that the two could be linked.

Any advice you are able to provide would be gratefully received.

Kind regards,


Dear B.,

1) It makes no statistical difference to the result whether you have one or both embryos transferred simultaneously.

2) You may have to face the probability that this cycle has a slightly higher chance of failure any way, because all the embryos you produced came from six/eight eggs which would all have been to same environment during ovarian stimulation. Our published research clearly shows that environment (which is not really under your control for much of the time) is one of the most important factors in IVF success or failure. Given that failure has already occurred twice with this group of fertilised eggs, this may tend to indicate less “good quality” in all the embryos.

3) Visual inspection, incidentally, is well nigh meaningless. As I have repeatedly explained elsewhere on this website, looking at embryos (or eggs) down a conventional microscope gives an extremely poor indication of viability, no matter what your clinical embryologist may maintain. Your embryologists are not being dishonest but not unreasonably they want you to think reasonably positively.

4) There is little evidence that growing these embryos on to the blastocyst stage before transfer will make any difference.

5) There is no statistically valid evidence that taking aspirin will improve the chance of implantation.

6) Avoid tests for natural killer T cells. They are not indicated, of no proven value, and are expensive. Save the money and consider making a donation to this charity which needs to raise income to support patients like yourselves and we make it very clear that the advice given is completely impartial and based on the best research evidence which is available.

Good luck with your transfer.

Best wishes,

Robert Winston