# The most important IVF success rate statistic you can’t find anywhere

If you’re planning to or already are undergoing IVF, one of the most important things you’ll want to know is how many cycles you’re likely to need before you hit the jackpot.

While commonly published “IVF success rates” can tell you the probability of success with each cycle or embryo transfer; these numbers fail to tell you what you really want to know.

You can now also take it a step further by using my IVF Success Rates Calculator which calculates what the actual IVF outcomes were for real women relevant to your similar situation (IVF success rates by age, specific fertility problems, your chosen IVF treatment method etc) using the most recently published population statistics for IVF.

But it’s an entirely different thing to know:

- What your success rates of IVF are over multiple attempts and
- If you are lucky enough to have enough embyros from a cycle, knowing your chances of having multiple kids from those embryos.

Now to make things really simple for you and because the math is complicated, I have put together two table’s to answer these two questions. I will explain throughout this post how they work and how to use them. They are a free resources and you can access them on my site by clicking on the link.

- Multiple Cycles IVF Success Rate Table – to figure out your chance over multiple attempts
- Multiple Births IVF Success Rate Table – to figure out your chances of multiple successful births using any number of embryos

I will explain in more detail below but you can essentially take the number you got using my IVF Success Rates Calculator to figure out your success rate on **a PER TRANSFER basis** and then plug it into the Multiple Cycles IVF Success Rate Table to then figure out your chances of success **over MULTIPLE CYCLES.**

You can also take the number you got from the IVF Success Rate Calculator and plug it into the Multiple Births IVF Success Rate Table to figure out your chances of multiple successful births using any number of embryos.

In this article, I’ve asked my math-nerd husband, David, to explain to you exactly what he explained to me when I wanted to know my chances of success after each successive IVF attempt. After reading this you will know exactly how to estimate the number of IVF cycles you’re likely to need. If you’ve seen my previous blog post on IVF success rates, or accessed my IVF success rate calculator, you’ll know that the success rate of IVF has become an area where David has some great statistical knowledge that can help couples interested in quantifying their success rate of IVF probabilities in the most useful way possible.

**Here’s David:**

When I first started looking at how to calculate IVF success rates for my wife, Kym, I realized it wasn’t as simple as I originally had thought. If the odds of having a successful birth using IVF for women of a particular age and health diagnosis is 33% per transfer, that means, odds-are that they will have a birth after three cycles or less right? But what are the odds? Part of me wants to say its 99%, but intuitively that just doesn’t quite seem right…

It turns out, that the answer is 70%. Let me explain…

## Why knowing the number of IVF cycles is important

At the time of writing this post, my wife had just been through her second round of egg retrieval for IVF which really knocked her about. We had our fertilized embryos frozen so she would have time to recover before her first transfer and now we wanted to know how many transfers she was likely to need before we are successful and she gets pregnant.

Since we’re an optimistic couple, we also hope to have more than one child over the next few years so understanding how far our clutch of 5 embryos is likely to go is also playing heavily on our minds. We know that having had this much trouble getting pregnant the first time, our second attempt to have a child at an older age with worse egg quality is probably quite unlikely.

If we want to have the option of having 2 or 3 kids, should we

do another egg retrieval now before she gets older and her egg quality diminishes further, or should we just get on with it and start our transfers since we want a baby so bad? It’s been four years now since we started trying and I have to admit our patience is running out.

## Should we do another round of retrievals?

## An evolution in what constitutes ‘cool’

Something awesome I only discovered in my early 30’s which would’ve been a great comfort to know when I was struggling to impress the cool girls in high-school, was that what women think is cool really changes as they start to think about having a family. My ability to get excited about math and science, and to use this interest to help us make informed decisions about our fertility choices is a prime example.

As you will have hopefully have discovered in the previous post relating to IVF success rates, I recently put on my SuperNerd outfit and spent some time learning and analyzing all I could to understand the relationship between per cycle and per transfer success rates of IVF and how this influences the number of IVF cycles needed to have a baby. Given that some women undergo ten or more cycles, I figured some others out there may find my latest analysis useful also.

## Things you must know about IVF success rates

This post is essentially a sequel to Misleading IVF Success Rates & The Numbers You Really Need To Know. In this post, I assume you are already familiar with the following core concepts outlined in this post:

- You understand the difference between per cycle, per transfer, and per woman IVF success rates by age probabilities.
- You understand that approximately 40% of women younger than 35 fail in their attempts at IVF, and for women over 40, it rises to over 80%.

The probabilities given in this post indicate the chances of success after each cycle of IVF for **women that will eventually be successful with IVF**. As described in Misleading IVF Success Rates & The Numbers You Really Need To Know, many women will not get pregnant or have a baby using IVF and hence they are excluded from this analysis.

### The math lesson you can skip about IVF success rates

## Binomial Distribution

Conducting an IVF cycle or transfer is an event with only two possible outcomes: success or failure. If we assume that the probability of success for each cycle or transfer is independent i.e. just because you failed the first time, doesn’t mean you’re chances of failing the second or third time are increased, then we can use binomial probability distribution to model the chances of success after various cycles. Binomial probability distribution is a statistical concept used to describe “trials” with a fixed number of possible outcomes like rolling a dice, or flipping a coin and hence is appropriate for answering the baby/no baby question (only 2 possible outcomes).

### A Worked Example

Understanding the theory is easier when we look at an example. If we assume that Kym has a per transfer probability of getting pregnant of 33% each time, the good Dr sticks one of our five embryo’s “back in”, then her probability of success after each transfer can be represented as shown in the figure below.

**Figure 1. Probability of pregnancy after each embryo transfer during IVF.**

The example shows that while the probability of Kym getting pregnant on her second transfer attempt is 22%, the probability of her getting pregnant in two or less attempts is 55% because she may get pregnant during either attempt 1 or attempt 2. The binomial probability distribution is the generalization of the calculation shown in the example.

## Confirming the Theory against Real-Life

Having a mathematical model is one thing, but knowing it works is another right? By comparing the results given by Macaldowie et. al (2014), who have published IVF results from clinics in Australia and New Zealand, with those determined using the binomial probability distribution, we are able to see that our model gives reasonable results as shown in the figure below.

**Figure 2. Comparison of IVF success rates predicted by the binomial distribution vs. actual results for women aged 35-39 as reported by Macaldowie et. al 2014.**

In the example shown, the binomial distribution estimate overstates the probability of success by a maximum of 3%, predicting a probability of success of 90% after four attempts, rather than the 87% observed in the reported data. This is an acceptably small error for our purposes and hence we consider the binomial distribution useful for making smarter fertility choices.

## Further support for the theory of binomial distribution

The figure below, reported by the CDC in 2012, also provides data to support the use of the binomial distribution for predicting the number of IVF cycles women are likely to need. One of the most important ‘simplifications’ of the binomial distribution is that it assumes that per transfer success rates do not change with the number of previous cycles undertaken or in other words, your chances of success on your third attempt, are the same as the chances on your first. From the figure below, it can be seen that this assumption is not strictly true as most women tended to have a slightly higher success rate on their first attempt. This phenomenon may partially explain the 3% error observed in Figure 2 above. However success rates seem to be relatively similar for second, third and fourth attempts supporting the use of the binomial distribution.

**Figure 3. Percentages of ART Cycles Using Fresh Non-donor Eggs or Embryos That Resulted in Live Births, by Age Group and Number of Previous ART Cycles, Among Women with No Previous Live Births, 2012.**

## In Summary – How to Calculate the Probability of Having a Baby after Multiple Cycles

So what this all means is that to find out what your general statistical chances of success are with IVF after two or more attempts (cycles or embryo transfers) you only need two pieces of information:

- What your per cycle or per transfer probabilities are – which you can get from the IVF Success Rates Calculator we have put together and:
- Multiple Cycles IVF Success Rate Table to complete your calculations of multiple attempts.

Take the number you get from the calculator and plug it into the Multiple Cycles IVF Success Rate Table. Simple! In the table, you will see that after three cycles, a woman like Kym, with a per transfer success rate of 33% will have a 70% chance of having a baby.

### Probability of Having Multiple Children from Multiple Embryos – Table #2

What’s super awesome about the binomial distribution is that it also can be used to estimate the probability of having more than one child from a known clutch of embryos (for people like Kym and I), or from a known number of retrieval cycles (for people that haven’t started yet).

For those of you wanting an idea of probabilities for this type of question, we have developed a second table, the Multiple Births IVF Success Rate Table, which is also available for free like the first table and lets you look at the probability of having two or three children after each success IVF cycle, for various per transfer success rates.

For women with frozen embryos in storage hoping to have more than one child, these tables will be particularly interesting. Do the same as you would for the Multiple Cycles Table and take the number you get from the IVF Success Rates Calculator and plug it into the Multiple Births IVF Success Rate Table.

I hope this information has helped empower you. If you have any questions, please comment below and best of luck with your smart fertility choices!

**David**

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