Day 7 blastocysts show disrupted gene expression in the ICM

A new study showed that day 7 embryos may develop more slowly because of stress and disrupted signaling in the inner cell mass, which might help explain their lower success rates.

Growing embryos to day 7 can help patients expand the number of blastocysts they get from an IVF cycle.

But even when they’re euploid and good quality, day 7 blastocysts don’t perform as well. Live birth rates are consistently lower than for embryos that become blastocysts by day 5 or 6. Why this is isn’t clear.

A new study by McCallie et al. (2025) set out to explore this, and compared gene expression in day 5 and day 7 euploid embryos, focusing on the ICM and trophectoderm. All of the euploids (40 in total) were age matched and grade matched to help isolate differences. The embryos came from patients under 40 with no severe diagnoses, and had similar clinical characteristics. All embryos were fair to good quality (≥4BB).

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Overall, more than 4,000 genes were expressed differently between day 5 and day 7 blastocysts, with the biggest changes seen in the ICM:

• In the ICM of day 7 embryos, the expression of 2,880 genes were significantly different from day 5.
• One of the biggest differences was in genes involved in calcium signaling, a key pathway that helps control cell communication and growth.
• Key genes involved in the primitive endoderm and epiblast (the parts of the ICM that eventually form the yolk sac and fetus) were differentially expressed in day 7 embryos. This could affect how well the embryo organizes itself, and disrupt the timing and coordination needed for healthy embryo development and implantation.
• There were also signs of metabolic stress. Day 7 embryos showed increased expression of genes linked to cell stress and cell death, and decreased expression of genes involved in embryo metabolism. This points to day 7 embryos being under more strain as they develop.
• They also found differences in the expression of genes linked to proteasome activity and epigenetic regulation, suggesting that protein turnover and gene expression control may be disrupted in day 7 embryos.

There were fewer meaningful differences in the trophectoderm of day 7 vs day 5 embryos.

Overall, this data suggests that embryos reaching the blastocyst stage on day 7 may be under more strain compared to day 5, particularly in the ICM. These embryos showed signs of stress, disrupted signaling, and changes in expression of genes that could affect their ability to implant and grow.

These differences might help explain why day 7 embryos have lower success rates, even when they are higher quality and are euploid. Other studies, referenced below, show that mitochondrial content might also influence slower blastocyst development.

However, this is still early research! The study didn’t look at implantation outcomes directly, since the embryos were destroyed for RNA analysis. So they don’t know if these molecular markers that they identified could be used to predict which embryos would implant and work.

Still, it gives an interesting look at the potential reasons day 7 embryos take longer to form a blastocyst, and could help guide future improvements in culture conditions or embryo selection.

Related studies

These additional studies were referenced by the authors of the paper and haven’t been covered on Remembryo. They may be helpful if you’re exploring this topic further. This section is available for paid subscribers.

Reference

McCallie BR, Denomme MM, Haywood ME, Trowbridge K, Hamm JM, McCarty KJ, Schoolcraft WB, Katz-Jaffe MG. Molecular disruptions to the future fetal tissue lineage are associated with delayed euploid blastulation. Fertil Steril. 2025 Jun 9:S0015-0282(25)00491-1. doi: 10.1016/j.fertnstert.2025.06.001. Epub ahead of print. PMID: 40499709.

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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.

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