Which Embryo To Transfer?

Embryologists use two primary, complementary systems to evaluate an embryo’s potential for leading to a successful pregnancy: morphological grading and preimplantation genetic testing.

A. Morphological Grading: The Art of Embryology

Morphological grading is a visual assessment of the embryo’s form and developmental progress under a microscope. It is a non-invasive, crucial daily checkup.

For Day 3 Embryos (Cleavage Stage):
We evaluate based on three criteria:

1. Cell Number: An ideal Day 3 embryo has 6-10 cells. Fewer may indicate slow development; more may indicate overly rapid, unstable division.

2. Degree of Fragmentation: This refers to tiny pieces of cellular material that are not part of the main cells. Lower fragmentation (less than 10%) is optimal, as high fragmentation can compromise the embryo’s viability.

3. Symmetry: We look for cells (blastomeres) that are of relatively equal size and shape. Significant asymmetry can indicate abnormal division.

For Day 5/6/7 Embryos (Blastocyst Stage):
This is the gold standard for transfer. We use the Gardner Blastocyst Grading System, which provides a score like 4AA or 3BC. It breaks down as follows:

1. Blastocyst Expansion Grade (The Number: 1-6):
   This measures the growth and expansion of the blastocoel (the fluid-filled cavity).

   • 1-2 (Early Blastocyst): Cavity is just forming.

   • 3-4 (Expanding/Expanded Blastocyst): The cavity is growing and the embryo is thinning. This is a common stage for transfer.

   • 5 (Hatching): The embryo is breaking out of its shell (zona pellucida). This is a very positive sign.

   • 6 (Fully Hatched): The embryo has completely escaped its shell.

2. Inner Cell Mass (ICM) Grade (The First Letter: A-C):
   The ICM is what will become the fetus. We assess its appearance.

   • Grade A: Many tightly packed, prominent cells. Excellent.

   • Grade B: Several loosely grouped cells. Good.

   • Grade C: Very few cells that are hard to distinguish. Poor.

3. Trophectoderm (TE) Grade (The Second Letter: A-C):
   The TE is what will become the placenta. It is critical for implantation.

   • Grade A: Many cells forming a cohesive, smooth layer. Excellent.

   • Grade B: Fewer cells, with a less organized appearance. Good.

   • Grade C: Very few, large, or degenerate cells. Poor.

Expert Insight: A 4AA or 5AA blastocyst is considered excellent. However, many babies are born from BB and even BC graded embryos. The trophectoderm (TE) is often given significant weight, as a strong placenta is essential for sustaining the pregnancy.

B. Preimplantation Genetic Testing for Aneuploidy (PGT-A)

While grading tells us about an embryo’s “fitness,” PGT-A tells us about its genetic “normalcy.”

• What it is: A biopsy of 5-10 cells from the trophectoderm is sent for genetic analysis.

• What it detects: Aneuploidy – an abnormal number of chromosomes. Examples include Trisomy 21 (Down syndrome) or monosomies (missing a chromosome). Aneuploid embryos rarely implant, lead to miscarriage, or can result in a child with a genetic condition.

• The Result: Embryos are classified as:

  • Euploid: Chromosomally normal. These have the highest chance of a successful, healthy pregnancy.

  • Aneuploid: Chromosomally abnormal. These are typically not recommended for transfer.

  • Mosaic: A mix of both euploid and aneuploid cells. The transfer of mosaic embryos is a complex decision, discussed in detail below.