Definition - What does Expanded Blastocyst mean?
An expanded blastocyst is a fertilized egg that has reached the blastocyst stage (around 5 days after fertilization) and further developed. As the blastocyst develops, the blastocoel cavity enlarges and the protective shell known as the zona pellucida thins. The degree to which this has occurred is said to be how expanded the blastocyst is.
The degree of expansion of the blastocyst is an important consideration in grading embryos. With in vitro fertilization (IVF), ideally only the best embryos, which are the ones assigned the best grades by an embryologist, are selected for transfer.
FertilitySmarts explains Expanded Blastocyst
Embryos are graded by embryologists to select the ones most likely of producing a successful pregnancy. The system for doing so varies by clinic but generally, involves scoring the different cell types within the blastocyst as well as the quality of the blastocoel. Another grade is assigned to designate how well the blastocyst has expanded.
Early on in development, the blastocyst is surrounded by the zona pellucida, a membrane still present from the egg. However, the blastocyst needs to break free of the zona pellucida in order to successfully implant in the uterine wall. As a blastocyst develops, it expands in size and the zona pellucida begins to thin. Around 5-6 days after fertilization, a healthy blastocyst escapes the zona pellucida or "hatches." Within 24 hours of hatching, the embryo typically begins to implant in the uterine wall. A blastocyst that is well expanded or has hatched is given a higher grade when selecting embryos for transfer.
If a blastocyst has not hatched on its own prior to transfer, assisted hatching may be performed in hopes of improving the chances of implantation.