Glossary · Definition · v2026.3
What Is Embryo Development?
Embryo development describes the stages an embryo goes through during the first days after fertilization.
Stages of Early Embryo Development
Day 0: fertilization produces a zygote. Day 1: the zygote divides into two cells. Day 2: four cells. Day 3: six to eight cells (cleavage stage). Day 4: the embryo compacts into a morula. Day 5 to 6: the embryo forms a blastocyst with an inner cell mass and trophectoderm. Each stage represents a developmental checkpoint where some embryos arrest.
Why It Matters in IVF
Understanding embryo development helps explain the attrition that occurs between fertilization and transfer. A cycle that produces ten fertilized eggs may yield only three to five blastocysts. This is not a laboratory failure but a reflection of normal biological selection. Embryo development rate and morphology at each stage inform grading decisions and transfer timing.
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This content defines terminology for educational orientation. It does not constitute medical advice.
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