What Does Embryo Grading Mean, and Can a Low-Grade Embryo Succeed?

Short answer: Embryo grading is a laboratory assessment of how an embryo looks and develops. It helps embryologists prioritize embryos for transfer or freezing, but it is not a genetic test and cannot predict pregnancy or live birth with certainty. A lower-grade embryo may still implant and result in a healthy birth.

What is embryo grading in IVF?

During IVF, embryologists monitor embryos as they develop in the laboratory. At the cleavage stage, they may assess the number and appearance of cells, how evenly the cells are dividing, and the degree of fragmentation. When an embryo reaches the blastocyst stage, usually on day 5 or day 6, the laboratory may also assess how expanded it is and the appearance of two important groups of cells.

Grading systems and laboratory policies can vary. A grade from one clinic may not mean exactly the same thing at another clinic, so the most useful explanation should come from the embryologist who evaluated your embryos.

What do the numbers and letters in a blastocyst grade mean?

A commonly used blastocyst grading method combines a number with two letters. The number generally describes the blastocyst's stage of expansion. The first letter usually describes the inner cell mass, which develops into the fetus. The second letter usually describes the trophectoderm, which contributes to the placenta.

For example, a grade such as 4AA may describe an expanded blastocyst with strong-looking inner cell mass and trophectoderm features. However, the grade must be interpreted using that laboratory's system and the day on which the embryo reached the blastocyst stage.

Does a high embryo grade guarantee success?

No. A higher morphological grade may be associated with a better chance of implantation compared with other embryos from the same cycle, but it cannot guarantee pregnancy or live birth. Outcomes are influenced by many other factors, including the age of the egg source, the embryo's chromosomal status, laboratory conditions, uterine factors, transfer protocol, and treatment history.

Embryo grading is best understood as a prioritization tool. It helps the laboratory and physician decide which embryo may be transferred first, rather than declaring that one embryo will succeed and another will not.

Can a low-grade embryo lead to a healthy birth?

Yes. Embryos with lower morphological grades can implant and lead to healthy births. Their average success rates may be lower than those of embryos with optimal morphology, but their chance is not zero. Letters and numbers alone should not be used to conclude that an embryo has no potential.

When no highly graded embryos are available, a clinic may still recommend freezing or transferring a lower-grade embryo based on its specific appearance, developmental timing, the patient's history, and the laboratory's policies. This decision should be individualized with the fertility team.

Is embryo grading the same as PGT-A?

No. Morphological grading evaluates how an embryo looks and develops. PGT-A analyzes a small sample of trophectoderm cells to estimate whether the embryo has the expected number of chromosomes. These assessments provide different information, and neither guarantees pregnancy or live birth.

An embryo with excellent morphology can have an abnormal PGT-A result, while an embryo with less favorable morphology may be reported as euploid. When several embryos have similar PGT-A results, morphology and developmental timing may still help a clinic determine transfer order. Read more in our guide: Is PGT-A Required for Surrogacy?

What should you ask your IVF clinic?

1. Which embryo grading system does your laboratory use?
2. On which day did each embryo reach the blastocyst stage?
3. What does each number and letter mean in our specific report?
4. What are your laboratory's criteria for freezing an embryo?
5. How does the grade affect the recommended transfer order?
6. If PGT-A was performed, how do you combine the genetic result with morphology?

What does embryo grading mean for surrogacy?

In a gestational surrogacy journey, the IVF clinic recommends which embryo to transfer based on the available clinical information. The goal is generally to select an appropriate embryo while reducing avoidable risks to the surrogate and baby. A concern about a lower embryo grade should not, by itself, lead to transferring more than one embryo, because multiple pregnancy carries additional medical risks.

If you are still preparing for a surrogacy journey, you may also find our guide on how many embryos you may need for surrogacy helpful.

The bottom line

Embryo grading is useful, but it is not a final verdict. A high grade does not guarantee success, and a lower grade does not eliminate the possibility of pregnancy or live birth. Ask your clinic to explain its grading system and each embryo's developmental history, then make decisions using the full clinical picture.

Planning embryo creation, egg donation, or surrogacy? Expecting.ai lets you explore surrogates, egg donors, sperm donors, IVF clinics, and fertility providers in one place.

Professional sources

• HFEA - Time-lapse imaging and embryo grading
• HFEA - Decisions to make about your embryos
• HFEA - Pre-implantation genetic testing for aneuploidy

This article provides general educational information and is not a substitute for personalized advice from a fertility physician or embryologist.