Egg Freezing Outcome Estimator

The Spring Fertility data uses the same method as Study B (by Doyle and colleagues). However, since most of the blastocysts we transfer have been previously tested for number of chromosomes, we assess the likelihood of obtaining a “chromosomally normal” blastocyst, rather than simply an untested blastocyst.

At Spring Fertility, we update our numbers every quarter based on the available data. This data set is representative of eggs thawed by December 31, 2023.

1. First, we evaluate the chance that a frozen egg will make it to a blastocyst stage embryo after warming – currently 37%. These data include 3,722 warmed eggs. The critical inputs are below:
   1. Thaw survival rate:  93%
   2. Successfully warmed (viable) egg to blastocyst development rate (this number will depend on the fertilization rate and embryo development rate): 37%. *We do note some differences in these two numbers based on age at the time eggs were frozen:
      • 41% of cryopreserved eggs “frozen” BEFORE a patient’s 35^th birthday fertilize and develop to blastocyst.
      • 36% of cryopreserved eggs “frozen” BETWEEN a patient’s 35^th birthday and 37^th birthday fertilize and develop to blastocyst.
      • 22% of cryopreserved eggs “frozen” AFTER a patient’s 37th birthday fertilize and develop to blastocyst.
2. Next, we estimate the percentage of these resulting blastocyst embryos that will be chromosomally normal (euploid) based on a woman’s age at the time her eggs are frozen.

*Please keep in mind that these data vary from clinic to clinic and even physician to physician, so if you are not a Spring patient and are using this tool, we recommend that you inquire with your doctor/clinic to understand the rates that their patients experience.

3. Lastly, we input the live birth rate per transfer of a chromosomally normal embryo. This will depend on the success of your clinics cryopreservation and frozen embryo transfer protocols and implementation. This is not dependent on the age of the woman receiving the transferred embryo. At Spring this number is currently 62%.

Steps 2 and 3 can be eliminated and you can substitute the live birth rate per blastocyst if your center does not test many embryos before transferring them.

The resulting output from our data result in live birth rates per cryopreserved oocyte as follows:

Doyle and colleagues have reported the next largest data set to date, including live birth rates after warming 1,283 vitrified and subsequently warmed eggs between 2009 and 2015. This is one of the largest published data sets available. While not every embryo created from these frozen eggs was transferred, the authors estimated the live birth rates as if every viable blastocyst stage embryo was transferred. As a proxy for older patients who had fewer embryos, they used their success rates from their IVF outcomes – they had previously demonstrated that blastocyst stage embryos from frozen eggs had at least as high a pregnancy and live birth rate as blastocyst stage embryos from women of the same age using fresh, never frozen eggs.  Their published outcomes involved the below calculations:

1. Calculate the number of live births from frozen eggs from women at various ages.
2. They still had embryos (blastocysts) remaining from the frozen eggs that were not transferred and refrozen, so they estimated how many of those frozen blastocyst embryos would result in a live birth based on their experience with blastocyst stage embryos frozen from women of the same age (at the time the eggs were retrieved). This estimate allowed them to approximate the efficiency per egg.
3. Since this number looked unrealistically high for women at age 41-42, they substituted the efficinecy of live birth per fresh oocyte at this age and applied this number to the eggs that survived freezing/warming.

This study lacked extensive data on eggs frozen from women who are older than 37 (only 96 eggs).  The 96 warmed eggs from women over age 37 yielded 5 babies. These numbers gave an unusually high efficiency for women over age 37 so the authors correctly adjusted their data to “predict” the efficiency using:

1. Their survival rate for frozen eggs.
2. The anticipated number of blastocysts that would develop from those eggs.
3. Their live birth rates using blastocysts derived from fresh (non-frozen eggs) at each age group.

They then assumed a binomial “normal” distribution to yield the predicted probability per frozen oocyte at each age group. Our calculator uses the same assumption (a binomial distribution) to arrive at the probability of 1 or multiple children based on the number of eggs and the probability of a live birth per egg.

They arrived at the following estimates for live birth efficiency per egg:

Cobo and colleagues is the largest data set of published outcomes from frozen/warmed oocytes. This study simply reported on the live birth rate per warmed egg in women of each age group.  We entered these estimates into a binomial calculator to arrive at the chances of having 1 or multiple live births based on the number of eggs used in each age group.

Interesting additional information:  the majority of these eggs were not tested for chromosome number. This study further reports no difference in outcomes based on amount of time samples were frozen (up to five years).  90.4% of warmed eggs survived the thaw and 1.4% of warming cycles had no eggs survive. These data yielded the following outcomes (the number of eggs included is represented on the right column).