The premise behind egg freezing is that a woman’s eggs decline in quality as they age, but this happens earlier than her ability to carry a pregnancy in utero. So we can use younger, frozen-and-thawed eggs in an older woman’s uterus and get a higher pregnancy rate than she could expect with her older eggs.
Egg freezing is talked about like it’s a brand new concept but, in reality, that’s not completely true. We’ll explain how egg freezing is almost the same process as a standard procedure called egg donation, except that it gives a woman the opportunity to be her own donor, with her younger self donating to her older self.
We know this is a bit confusing, so we’ll explain more in the chapters below.
Let’s first look at the premise that as women age, their egg number decreases, and the quality of those eggs declines too.
Below is data from one clinic that reflects the number of eggs retrieved per cycle from women who froze their eggs. As you can see, older women typically have fewer eggs retrieved. While this data may vary between clinics, it is a well acknowledged phenomenon.
Below is data showing how the quality of women’s eggs decreases over time. The proxy for quality here is whether embryos (created from eggs) were chromosomally normal. We should note that this data was collected on IVF patients, who’s fertility decline may not reflect the general population.
An interesting thing about a woman’s biology is that the quality of her eggs generally declines faster than the quality of her womb, or her ability to carry a pregnancy. This is why older women can reliably have children using donated eggs (which we cover below).
So the premise, and hope, of egg freezing is that extracting and freezing eggs while a woman has more high-quality eggs will allow her to use them later and have a child if she has trouble conceiving on her own.
Egg freezing leverages the disparity in time between when most women’s egg number and quality goes down and when women physically can no longer carry a pregnancy in utero.
Egg donation is similar to egg freezing. Egg donation is when a younger woman has her eggs retrieved and inseminated and in turn the resulting embryos are transferred to an older woman. When a 42-year-old woman tries to conceive using her own eggs with IVF her rate of success is 4%. When she uses frozen eggs from a younger donor, her rate of success jumps to 60%.
Egg donation works and, for this reason, retrieving eggs from younger women and transferring them to older women has grown in popularity.
Egg freezing is the same as egg donation, but in the case of egg freezing, your younger self is donating frozen eggs to your older self. So the premise and technology of egg freezing works, but not evenly for all patients.
The likelihood a woman’s frozen eggs will lead to a live birth is a function of the age at which she froze her eggs and the number of mature, high-quality eggs she froze (more on this later in the course).
Below is data from one Spanish center showing live birth rates for women who froze their eggs and returned back to use them. The Spanish center breaks down the results by the age at which the woman froze her eggs and how many eggs were stored. However, because this study was done at a single center, on a small sample set (300 women), using crude age brackets, we cannot say if these rates of success are generalizable to most patients treated at most clinics.
A model built to predict a woman’s likelihood of success from Brigham & Women’s Hospital in Boston is far more optimistic. But the model is not based on the success rates of actual egg freezing patients, and so it is mostly theoretical. The model characterizes patients similar to egg freezers and then extrapolates from a set of defensible assumptions. Left to our own devices, we’d more heavily rely on the Spanish data.