One of the biggest areas of confusion for egg freezing patients is: “How do I know what counts as success?” With most fertility treatments, there is a definite and fairly quick feedback loop: did last month’s treatment result in pregnancy, and nine months later did that pregnancy result in a healthy live birth? Egg freezing success, on the other hand, is far more ambiguous, at least in the near- term.
When eggs are frozen, the quality is unknown. It won’t be clear how many of them will thaw, fertilize, become a chromosomally-normal blastocyst, implant, and yield a healthy pregnancy.
With all of these hurdles that your eggs need to overcome, how do you know today if you’ve frozen enough eggs that at least one will result in a baby one day?
There are no guarantees with egg freezing, but we do have some statistics and data. The two best variables to focus on are the age at which you freeze your eggs, and the number of mature eggs frozen.
Age of Egg Freezer: Less is More
To have a healthy, live birth, you need to start with a chromosomally-normal egg. The same is true for a batch of frozen eggs: good eggs are a requirement to lead to a good outcome. Younger women typically produce higher quality eggs.
Number of Eggs: More is More
Generally speaking, the more eggs a woman freezes, the higher the likelihood she will have frozen at least one good egg. Younger women have more available eggs, so at the end of the egg freezing process, they are able to freeze more. The opposite is true for older patients, who have fewer eggs, and get fewer from each cycle.
Mix of Eggs
Likewise, the next variable is what proportion of the eggs you do retrieve, are good. Typically, younger women have a higher proportion of good eggs, as evidenced by the chart below. Embryo quality is a stand-in for egg quality, and clearly the ratio of good eggs decreases with age.
1 + 1 = 3
When we add the effects of egg quality and volume together, younger patients clearly have far higher success rates with egg freezing. Older women need to collect far more eggs to feel confident in a successful outcome.
A recent study out of Europe looking at 140 women who electively froze their eggs and returned to use them, gives us a glimpse into the impact of age and egg volume.
Right off the bat, you can see that being younger and storing more eggs was a major determinant of success. Women who stored 10 eggs after the age of 35, had a 30% success rate. Women under 35 with the same number of eggs had twice that success rate. That’s a big step up, and we’re not done. For women under 35 who managed to bank not 10 eggs, but 20 eggs, their success rate jumped to 80%.
Ahead of Ourselves
It’s tempting to look at this study and make a guess about how likely your eggs are to work. The reality is, these buckets are broad (a 36-year-old and 46-year-old aren't equally fertile), the sample size could be bigger, and there could be other variables at work. This is probably directionally accurate, but not worth carving in stone.
A Predictive Mode on Number of Eggs Frozen and Live Birth Rate
A team at Brigham & Womens in Boston decided to build a prognostic model to answer the same question: chances of success based on the woman's age at retrieval and how many mature eggs were retrieved. The dataset was not based on egg freezers, but similar sets of women who'd been treated at a Brigham for non-ovarian reserve related issues (e.g. male factor infertility or blocked tubes). The model relies on a number of tested assumptions on freeze, thaw, fertilization, blast & euploid rates.
Getting Comfortable Being Uncomfortable
For all of the time and money a woman invests in egg freezing, it often doesn’t deliver the confidence that patients want. That lack of confidence affects some patients. Our data shows patients will often do a second cycle, or even a third, to help drive up the theoretical probability of a live birth. As you can see in the case of women over 36, those extra cycles, and extra eggs, may or may not change the odds – there are certainly diminishing returns to continued cycles. But the incremental costs are undeniable.