IVF studies often report success rates per retrieval (or per patient) or per transfer.
Per transfer only includes patients who had an embryo transferred. Patients who never reached transfer (for example, because no embryos developed or no embryo was suitable for transfer) are not included.
Per retrieval includes everyone who started a retrieval cycle, even if they never had a transfer. In studies where each patient only had one retrieval, per patient and per retrieval mean the same thing.
For example:
- 320 retrievals → 100 embryo transfers → 58 live births
- Per transfer: 58/100 = 58%
- Per retrieval: 58/320 = 18%
The per transfer rate is useful if you already have an embryo to transfer. The per retrieval rate is useful when deciding whether to start a treatment or comparing different IVF strategies because it includes everyone who began treatment.
Synonyms:
per retrieval, per transfer, per cycle