What clinicians actually look for when they grade embryos, what the numbers mean, and what they don’t.
During your IVF journey, you will receive updates from our embryology team that may use technical terms you’re not familiar with yet. Understanding this language can give you valuable insights into the developmental progress of your eggs and embryos.
Egg Maturity: GV, MI, and MII
When your eggs are retrieved, the embryologists assess their level of maturity. Egg maturity is divided into three stages: GV, MI, or MII.
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GV (Germinal Vesicle): The egg is immature. These eggs cannot be frozen for future use or fertilized with sperm.
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MI (Metaphase I): The egg is maturing but is not yet ready for fertilization.
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MII (Metaphase II): The egg is fully mature and ready to be frozen or fertilized by sperm.
Fertilization: What is a 2PN?
The day after an egg retrieval or egg thaw, the embryologist checks the eggs for signs of fertilization.
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2PN (Two Pronuclei): This is the term for a successfully fertilized egg. You can see two small circles (pronuclei) inside the egg: one from the sperm and one from the egg.
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1PN or 3PN: This means one or three pronuclei were seen. If they develop into blastocysts and are determined to be chromosomally normal after PGT-A testing, they may still be suitable for future use.
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OPN: This means the egg didn’t fertilize.
Why Don’t All Fertilized Eggs Become Blastocysts?
On average, only about half of fertilized eggs develop into blastocysts (the stage ready for freezing, transfer or biopsy). This percentage decreases as the age of the egg increases.
The main reason why some fertilized eggs don’t become blastocysts is because they are genetically abnormal (aneuploid). This is a natural phenomenon that occurs both in IVF and inside the body.
The Twig Embryo Grading System
Embryos that reach the blastocyst stage are assigned a grade based on their appearance, or morphology, under the microscope.
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Purpose: Grading helps the team decide which embryos can be biopsied and/or frozen, and which embryo to prioritize for transfer if multiple embryos are available.
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The Code: The grade uses a number and two letters:
The Number: Represents the degree of development and expansion.
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First Letter (A, B, C or D): Describes the quality of the inner cell mass (the cells that become the embryo/fetus).
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Second Letter (A, B, C or D): Describes the quality of the trophectoderm (the cells that become the placenta).
It’s important to note that embryo grading is just one piece of data. Even lower-graded embryos can and do result in healthy pregnancies, especially if they are found to be euploid after PGT-A.
AI-Enabled Insights and Live Embryo Viewing
CHLOE™ is the world’s first transparent AI-based decision support tool for embryology. It offers real-time insights into embryo development, constantly monitoring and evaluating with the support of millions of data points.
This AI-powered technology helps our embryologists make informed, evidence-based decisions. Alongside CHLOE™, we’ve integrated MyEmbryoViewer, allowing you to watch a secure, time-lapse video of your embryos developing in real time.
Ready to explore your options? Click here to book a complimentary egg freezing or family building consult. No referral required.
