PGT A

PGT-A samples trophectoderm (TE) cells, not the inner cell mass (ICM).

• TE
  • Forms placenta and membranes
  • Higher tolerance for aneuploidy
  • Actively proliferating, genomically unstable early on
• ICM
  • Forms the fetus
  • Under stronger selection pressure
  • More likely to normalize via selection and apoptosis

So PGT-A is placental genetics at blastocyst stage, not fetal genetics.

This explains:

• Mosaic calls
• Chaotic calls
• False positives
• Normal babies after “abnormal” PGT-A

Origins of mosaicism

Most mosaicism detected on PGT-A is post-zygotic mitotic error, not meiotic nondisjunction.

• Anaphase lag
• Mitotic nondisjunction
• Chromosome mis-segregation during rapid cleavage

This creates lineage-restricted mosaicism:

• Abnormal cells preferentially populate TE
• Normal cells preferentially populate ICM

This is NOT theoretical & WELL documented in:

• CVS vs amniocentesis discordance
• Placental confined mosaicism (CPM)

Interpreting PGT-A result categories

1. Euploid

• All tested TE cells appear balanced
• Still not “guaranteed normal”
• Resolution limits apply (segmental <5–10 Mb may be missed)

Implantation potential: Highest

2. Aneuploid (uniform)

• Same whole chromosome gain/loss across sampled TE cells
• Often meiotic in origin
• But even here:
  • TE ≠ ICM
  • Trisomy rescue is possible (more below)

Implantation potential: Low but not zero

3. Mosaic

Typically reported as 20–80% abnormal signal.

What mosaic really means:

• Mixed cell population in the biopsy
• Or technical noise amplified by WGA
• Or true confined placental mosaicism

Important nuance:

• Low-level mosaic (20–40%)
  • Often TE-limited
  • High chance of normal ICM
• High-level mosaic (40–80%)
  • More likely true embryo-wide involvement
  • Still not deterministic

Implantation potential:

• Lower than euploid
• Much higher than uniform aneuploid
• Chromosome-specific effects matter (16, 22 worse than 15, 21)

4. Chaotic / complex aneuploidy

• Multiple whole chromosomes and segments gained/lost

This is the most misunderstood category.

What “chaotic” usually reflects:

• Early cleavage-stage genomic instability
• Mitotic catastrophe in TE lineage
• WGA amplification artifacts exaggerating noise

Crucial insight:

Chaotic TE does not necessarily mean chaotic ICM

True genetic makeup of the fetus

Amniocentesis beats PGT-A

• Amniocytes derive from epiblast
• They reflect fetal lineage
• This is why:
  • CVS can be abnormal
  • Amnio can be normal
  • Baby can be normal

Trisomy rescue: the great normalizer

What is trisomy rescue

Loss of one chromosome from a trisomic cell to restore disomy.

Occurs:

• Early post-implantation
• More commonly in ICM than TE

Outcomes:

• Normal disomy
• Uniparental disomy (UPD)
• Persistent mosaicism

This explains:

• Normal fetuses after trisomic embryos
• Placental mosaicism with normal baby
• UPD syndromes after “self-correction”

Chromosome	Rescue likelihood	Clinical implication
16	High	Common CPM, often normal fetus
22	Moderate	Risk of growth issues
15	Moderate	UPD risk (Prader–Willi / Angelman)
21	Low	Persistent trisomy more likely
13, 18	Low	Poor developmental tolerance

This is why trisomy 16 mosaic embryos sometimes do surprisingly well.

Implantation potential:

Determinants stronger than PGT-A label

• Endometrial receptivity
• Embryo metabolic competence
• Mitochondrial health
• Synchrony between embryo and uterus

PGT-A mostly predicts:

• Likelihood of implantation
• Not certainty of fetal outcome

Rate-limiting step is attachment and heartbeat

Once implantation succeeds and heartbeat establishes, selection has already happened.

Why PGT-A overcalls abnormality

Technical contributors

• Whole genome amplification bias
• Low DNA input
• GC-content variability
• Bioinformatic smoothing thresholds

A 5-cell TE biopsy represents:

• <1% of the embryo
• From a lineage not destined to become fetus

False positives are EXPECTED.

Key
“PGT-A tells us about implantation potential, not the final genetic makeup of the baby.”

Invasive prenatal testing still matters

• Even after euploid PGT-A:
  • De novo CNVs
  • Segmental imbalances
  • Mosaicism resolution

Amniotic CMA remains the gold standard for fetal genetics.