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Interpreting Fertility Success Rates

By: Dr. Dan Nayot, MD, MS.c, FRCSC, REI

Success rates matter. Both patients and fertility clinics have the same final goal in mind of having a healthy child – but how to interpret them can be confusing. Allow me to expand…


First, let’s define both terms:

Success” – This may be different for each patient. Is just being able to stimulate well so that you can retrieve eggs considered a treatment success? For some it’s certainly a big milestone. What about a BFP? Some would say so, even if it results in a miscarriage. Should only a live birth be considered a success? Some in our field suggest we should only consider a healthy single live birth as truly successful from a quality control perspective (twins would not be considered a successful outcome in this context).

Rate” – Recall your fractions from high school? Numerator / denominator. The numerator is whatever we choose as our measure of success. The denominator is the whole group. This is where it gets tricky. Which group? Everyone that walks into your office? Everyone that started the IVF cycle? Only patients that had embryos available for transfer?

Success rates are very important, but can be misleading and misinterpreted.

Let’s start with a common misconception – the term “cumulative pregnancy rate”.  Assume you were notified you had a 50% cumulative success rate with an IVF cycle. From a doctor’s perspective that would mean that if you eventually transferred all the embryos that were generated from that IVF cycle, 50% of patients would be pregnant. This may take several embryo transfer attempts over several months, and should not be misinterpreted as 50% chance of success with each embryo transfer – i.e. 50% chance of getting pregnant every embryo transfer. Clearly doctors don’t know what the end result of an IVF cycle will be (but we can predict with some certainty) – it could result in no eggs being retrieved, no eggs being fertilized or no embryos available to be transferred (0% success rate for an unfortunate patient).

Clinic A has an 80% pregnancy rate! This is fantastic, and I would recommend a referral to clinic A. But let’s look deeper.

Are you suggesting that 8 of to 10 people who get a referral to Clinic A will have a baby? Even among patients that are 44 years old? Even those that have failed say 3 IVF cycles in the past? Or do you mean, 80% of patients within a certain group or scenario will succeed? That’s still impressive, but is it applicable to every patient looking for a successful fertility clinic?

If clinics are being judged by their success rates (and it IS important, but only part of what makes an IVF center world-class), then there is clearly an incentive for IVF centers to demonstrate the highest rates.


Here are some ways clinics could (not should) improve their “success rates”


Transfer more embryos!

Your pregnancy rate per embryo transfer will certainly improve, but so will the possibility of multiples along with their known risks to the mother and babies (not generally reported with the success rates).  Complication rates are equally important but are not routinely reported.

Don’t allow patients unlikely to succeed (poor prognosis) to undergo an IVF cycle.

IVF is far from a guaranteed treatment. For example, if your doctor believes your chance to succeed is < 10% per IVF attempt, should they not permit you to undergo treatment? Many clinics have an age cut-off and won’t allow patients beyond this age group to undergo an IVF cycle since the chance of success is dramatically lowered. Only treating patients that are more likely to succeed will certainly increase the success rates.

Don’t allow embryos to be transferred unless they reach a certain grade.

Recall that even low-grade blastocysts have the potential to result in a pregnancy.  Clinics that will not transfer or even freeze low quality blastocyst (since the chance of success is lower) will certainly have a higher pregnancy rate per embryo transfer.  Some clinics may request that all embryos undergo mandatory PGS testing (with its added cost) because the clinic will only transfer chromosomally normal (euploid) embryos.

Bank embryos and choose the best one.

We will perform three IVF cycles and transfer the highest quality embryo. I’m not advocating against this in the correct clinical context, but would this be a 100% success rate (live birth / embryo transfer) or a 33% success rate (live birth / IVF attempt)?


Of course, success rates are a measure of quality control and clinics are required to report many variable outcomes to their governing body. However, in Canada these results are not made public on an individual-clinic basis – as a measure to protect patients from clinics being pressured to out-perform each other.  Reporting success rates then turns into a form of advertising which could be dangerous in terms of both misleading patients who focus on the outcomes instead their best interests.

Success rates matter, but please educate yourself in how they’re being defined and whether or not a statistic applies to your personalized situation.

As the old saying goes, if it’s too good to be true, it probably is.