Blastocyst
A blastocyst is an embryo five to six days after fertilisation, when it has formed a fluid-filled cavity and two distinct cell groups.
A blastocyst is an embryo five to six days after fertilisation, when it has formed a fluid-filled cavity and two distinct cell groups.
Why it matters
Reaching this stage is a meaningful developmental test: the embryo’s own genome has taken over. That is why many IVF laboratories culture to day 5 when there are several embryos to choose between — the blastocysts that form have pre-selected themselves.
In your treatment
Whether your transfer happens at day 3 or day 5 should follow your embryo numbers, not a fixed clinic habit. Ask what the laboratory recommends and why.
Definitions explain terms; they do not interpret your individual results or replace a consultation.