Researchers in a 2022 study give a detailed look at the number of retrieved eggs, mature eggs, fertilized eggs and blastocysts formed at each age, and find that blastocyst conversion rates in good prognosis women don’t decline until age 41.
After fertilization, each embryo has a chance to become (or convert into) a blastocyst, and this is given by the blastocyst conversion rate. It’s not clear whether or not the blastocyst conversion rate varies by age.
Check out my complete guide to embryo grading and success rates to learn more about embryo development, grading and success rates.
Romanski et al. (2022), in their retrospective study, compared blastocyst conversion rates and other IVF outcomes in mostly good prognosis women who performed their first freeze-all cycle at a USA-based IVF center between 2014 and 2020. A total of 3362 women were included in the analysis, ranging in age from ≤30 to ≥44.
In this study they based their blastocyst conversion rate on the number of “usable blastocysts” that developed. After contacting one of the authors, this was clarified as day 5 and 6 embryos without CC grades (so no day 7 embryos and no CC embryos).
Note that this study reported their data as median values rather than averages. To get the median, the values are ordered from smallest to largest and the middle value is taken. This is done to avoid the problem of outliers or skewed distributions. As an example, if there were 5 women with 10, 10, 11, 12 and 35 eggs retrieved, the median would be 11 while the average would be 16. In this case the median better represents the “typical” egg retrieval number compared to the average.
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Number of retrieved eggs, mature eggs, fertilized eggs and usable blastocysts decreased with age
They found that the median number of eggs that were retrieved, mature and fertilized decreased with age, as shown below. In all cases, this was statistically significant between women ≤30 and and ≥33 (so there were only differences starting at 33).
They also found that usable blastocysts decreased with age, which was significant in women ≤30 and ≥35. As indicated above, “usable blastocysts” didn’t include day 7 embryos, or embryos with a CC grade.
Egg maturation and fertilization rates mostly unaffected by age
There were no differences in egg maturation rates until ≥44 where there was a significant increase of 4.4% compared to women ≤30. The median for the whole group was 81.8%.
There were also no differences in fertilization rate until ≥44 where there was a significant decrease of 5.8% compared to women ≤30. The median for the whole group was 81.8%.
Blastocyst conversion rates decrease at age 41
Next they calculated the usable blastocyst conversion rates, which was based off of the median number of fertilized eggs that went on to form “usable blastocysts” defined earlier. They found a decrease with age that was only statistically significant from 41 onward (compared to women ≤30).
In other words, there was no difference in blastocyst conversion rates until 41 on.
Conclusions
This study found a steady decline in the number of retrieved eggs, mature eggs, fertilized eggs and usable blastocysts with age. Egg maturation rates and fertilization rates didn’t drop until age 44, while blastocyst conversion rates started to drop by 41.
Because of this, the authors state that poor egg maturation or fertilization rates shouldn’t be attributed to age, but instead other factors should be considered like stimulation protocol, timing of trigger, or egg/sperm quality. However, this study mostly involved good prognosis women and it’s not clear if this can be extended to poor prognosis patients.
The major limitation of this study is that it included mostly good prognosis women and didn’t include day 7 or CC graded blastocysts. Previous evidence indicates that these embryos work, albeit with a reduced success rate, and it’s likely that including these embryos would increase the blastocyst conversion rate.
Reference
About Embryoman
Embryoman (Sean Lauber) is a former embryologist and the founder of Remembryo, an IVF research and fertility education website. After working in an IVF lab in the US, he returned to Canada and now focuses on making fertility research more accessible. He holds a Master’s in Immunology and launched Remembryo in 2018 to help patients and professionals make sense of IVF research. Sean shares weekly study updates on Facebook, Instagram, and Reddit regularly. He also answers questions on Reddit or in his private Facebook group.
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