Blastocyst and pregnancy rates of day 3 embryos based on cell number, fragmentation

Researchers in a 2022 study found that day 3 embryos with >6 cells and <20% fragmentation are the most likely to become blastocysts and lead to a clinical pregnancy.

Day 3 embryo quality can be evaluated in terms of the number of cells, the amount of fragmentation and the relative size of each cell (symmetry). If you’re unclear on these terms or how day 3 embryos are graded, you can check out my post on grading day 3 embryos.

Liu et al. (2022), in their retrospective study, examined IVF outcomes of 2,517 day 3 embryos at a single IVF center in China between 2017 and 2020. Day 3 embryos were divided into several groups based on the number of cells, their symmetry and the amount of fragmentation:

6-10 cells; symmetrical, <20% fragmentation
6-10 cells, uneven cells, ≥20% fragmentation
>10 cells, symmetrical, <20% fragmentation
>10 cells, uneven cells, ≥20% fragmentation
<6 cells

For simplicity I won’t mention symmetry in the text/graphs below as it corresponds to the fragmentation percent in all cases.

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Day 3 embryos with ≥6 cells and low fragmentation are most likely to become blastocysts

The researchers examined the ability of day 3 embryos to convert into blastocysts and high quality blastocysts (≥4BB). In both cases, day 3 embryos with ≥6 cells and <20% fragmentation were the most likely to convert, and the other three groups were all significantly lower with the <6 cell group having the lowest potential.

Blastocyst conversion rates for day 3 embryos based on cell number and fragmentation — *Note that “n” refers to the sample size, or the number of day 3 embryos*

Blastocysts from day 3 embryos with ≥6 cells and low fragmentation have the highest pregnancy rates

Day 3 embryos in the different groups were cultured to blastocyst and then frozen for embryo transfer (mainly single embryo transfers). The researchers compared outcomes for day 5 and day 6 embryos, although each group was relatively small so they combined the two days.

Clinical pregnancy rates were highest from blastocysts that came from day 3 embryos with 6-10 cells and <20% fragmentation, which was found to be statistically significant.

*Note that “n” refers to the total (day 5 + day 6) sample size, or the number of transfers.*

Conclusions

This study found that day 3 embryos with ≥6 cells and <20% fragmentation were most likely to convert into blastocysts, including fast growing day 3 embryos with >10 cells. Single blastocyst transfers from day 3 embryos with 6-10 cells and <20% fragmentation had the highest clinical pregnancy rates.

Slow growing day 3 embryos with <6 cells had the lowest chance of converting into blastocysts, but once they did they had comparable clinical pregnancy rates with the other groups.

All of these embryos studied here had potential to convert into a blastocyst and lead to a clinical pregnancy, so the authors suggest that extended culture should be considered with low quality day 3 embryos before discarding.

Reference

Liu J, Zhou Y, Tong L, Wang X, Li Y, Wang H. Developmental potential of different embryos on day 3: a retrospective study. J Obstet Gynaecol. 2022 Oct;42(7):3322-3327. doi: 10.1080/01443615.2022.2125291. Epub 2022 Sep 23. PMID: 36149236.

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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.