Embryo compaction

A “compacted” embryo is what embryologists call a blastocyst that is squished up into a tight wad of cells. It can also be referred to as a “collapsed” or “contracted” blastocyst.

A fully expanded embryo and a compacted embryo. As embryos collapse or contract they become compacted.
A fully expanded embryo on the left and a compacted embryo on the right.

This happens because blastocysts contain a cavity called a “blastocoel” that’s filled with water, and when they lose this water the blastocyst deflates like a balloon would — causing it to become compacted/collapsed.

outside of the blastocyst showing the trophectoderm cells and inside showing the blastocoel and inner cell mass (ICM)
Modified from CNX OpenStax, CC BY 4.0

This can happen naturally, but blastocysts can also be artificially collapsed before freezing to remove water from the blastocoel. How well an embryo re-expands is linked to success rates.

More details in my post What is embryo compaction?

Synonyms:
re-expanded, re-expansion, re-expands, compacted, embryo collapses, embryo collapse, expansion and contraction