An expanded blastocyst is a zygote that has developed for around 5 days and has started the process of differentiation. There are three main components to a blastocyst. The inner cell mass layer of cells develops into the fetus, the trophectoderm or trophoblast develops into the placenta and the blastocoel is an empty cavity of fluid.
As the blastocyst develops, the blastocoel cavity enlarges with fluid which causes the protective shell known as the zona pellucida to thin. The degree to which this expansion has occurred is said to be how expanded the blastocyst is.
The degree of expansion of the blastocyst is a 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
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 have cell to cell interaction with the uterine lining and successful implant.
As a blastocyst develops, the blastocoel 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.
Embryos are evaluated and graded by embryologists to predict the embryos most likely of producing a successful pregnancy. The coding system for documenting grades varies by the clinic but generally, involves scoring the different cell types within the blastocyst as well as the quality of the blastocoel.
The degree of expansion, or how much fluid the embryo has taken into its cavity, is typically assigned a value between 1 and 6.
Cavity fills less than half of the volume of the embryo
Cavity fills more than half the volume of the embryo
Full blastocyst, cavity fills the embryo
Expanded blastocyst, cavity larger than the embryo, thinning of zona pellucida
Hatching out of the shell
Hatched out of the shell
The Center for Disease Control (CDC) also requires reporting the grade of each embryo transferred using a “Good”, “Fair”, and “Poor” and a descriptive system. These evaluations by embryologists are strictly regulated and each embryologist is tested twice a year to make sure they meet national standards.
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.
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Elgindy E, et al.
Day 5 expanded blastocysts transferred on same day have comparable outcome to those left for more extended culture and transferred on day 6.