Plan
Understanding success rates
Success-rate figures can easily mislead. Here is what a meaningful number must tell you — and why no figure can predict your own outcome.
What a meaningful figure must state
Whenever you see a success rate, look for all of the following. If any is missing, the figure is hard to trust:
- Population — who does it describe (for example, an age band)?
- Denominator — success per what? Per cycle started, per egg collection, or per embryo transfer are very different.
- Treatment type — which treatment does it cover?
- Reporting period — which years does the data come from?
- Source and methodology — who produced it, and how?
Why “per transfer” can look better than “per cycle”
A rate measured “per embryo transfer” leaves out cycles that never reached transfer, so it can look higher than a “per cycle started” rate for the same clinic. Neither is wrong, but they answer different questions — so only compare like with like.
Reliability ranges
The HFEA publishes a reliability range around each clinic's figure. Larger clinics, which carry out more cycles, have narrower and more reliable ranges. A small difference between two clinics often falls within these ranges — meaning it may not be a real difference at all. The HFEA also notes that most clinics are broadly comparable, and that differences are frequently explained by the different patients each clinic treats.
Where reliable data comes from
National registries and regulators publish standardised outcome data — for example theCDC in the US and theHFEA in the UK, whose public register reports results as births per egg collection and per embryo transferred. These are more comparable than a single clinic’s marketing figure.
Our choosing a clinic guide puts this into practice.
Sources
- Choose a fertility clinic — HFEA (accessed 19 July 2026)
- National ART Summary — CDC (accessed 19 July 2026)