Per retrieval vs per transfer

When outcomes are given “per transfer”, this refers to the number of women who had success per embryo transfer, whereas “per retrieval” is divided by the number of retrievals. With “per transfer,” women who didn’t have anything to transfer weren’t included (ie. maybe they had aneuploid embryos, or embryos arrested, etc).

This is often done for PGT-A and the per transfer stat is usually larger than the per retrieval statistic.

For example, there’s 320 retrievals, 100 euploid transfers and 58 live births…

• Per transfer: 100 euploids transferred, 58 live births = 58% live birth per transfer.
• Per retrieval: 320 retrievals, 100 had euploids to transfer (and 220 didn’t), resulting in 58 live births = 58/320 = 18% live birth per retrieval. Here we’re including all the retrieval cycles that failed to have a transfer.

The per transfer statistic is useful when you know you have euploids to transfer, and the per retrieval statistic is useful when you’re just starting out with the retrieval and don’t know if you’ll get euploids or not.