Confined Placental Mosaicism (CPM)

1. Confined placental mosaicism (CPM) means:

• The placenta has a chromosomal abnormality
• The fetus may be normal, abnormal, or mosaic
• The abnormal cell line is "confined" predominantly or entirely to placental tissues

2. Placental anatomy matters

Placenta has two genetically sampled compartments:

• Cytotrophoblast (CTB) → sampled by NIPT and CVS short-term culture
• Mesenchymal core (MSC) → sampled by CVS long-term culture

The fetus comes from the inner cell mass (ICM).

Where the abnormal cells sit determines:

• NIPT result
• CVS result
• Fetal outcome

3. The three classical types of CPM

CPM Type 1 – Cytotrophoblast-only mosaicism

Where is the abnormality?

• Present in cytotrophoblast
• Absent in mesenchyme and fetus

How it arises

• Usually post-zygotic mitotic error
• Occurs after trophoblast–ICM separation

What you see

• NIPT: positive
• CVS short-term culture: abnormal
• CVS long-term culture: normal
• Amniocentesis: normal

Frequency

• Most common type
• Accounts for ~50–60% of CPM cases

Clinical implications

• Fetus genetically normal
• Usually normal growth
• Lowest risk of adverse outcome

Key

False-positive NIPT is most often CPM type 1

CPM Type 2 – Mesenchymal core-only mosaicism

• Present in placental mesenchyme
• Absent in cytotrophoblast
• Fetus usually normal

How it arises

• Often post-zygotic mitotic error
• Occurs slightly later, after some lineage commitment

What you see

• NIPT: normal
• CVS short-term culture: normal
• CVS long-term culture: abnormal
• Amniocentesis: usually normal

Frequency

• Least common type
• ~10–20% of CPM

Clinical implications

• NIPT misses it
• Placental function can be impaired
• Higher association with:
  • FGR
  • Preeclampsia

"Silent CPM" you only detect if CVS is done and cultured long-term.

CPM Type 3 – True placental mosaicism (both layers)

Where is the abnormality?

• Present in:
  • Cytotrophoblast
  • Mesenchymal core
• Fetus may be:
  • Normal
  • Mosaic
  • Fully aneuploid

How it arises

• Often meiotic error with partial trisomy rescue
• Or very early mitotic error before lineage separation

What you see

• NIPT: positive
• CVS both cultures: abnormal
• Amniocentesis:
  • Normal (if rescue successful)
  • Mosaic
  • Abnormal

Frequency

• ~20–30% of CPM

Clinical implications

• Highest risk group
• Strong association with:
  • FGR
  • IUFD
  • Preeclampsia
  • Placental insufficiency

4. Meiotic vs mitotic origin and why it matters

Mitotic CPM

• Occurs after fertilization
• Usually mosaic
• Often type 1 or 2
• Lower fetal risk

Meiotic CPM

• Originates in gamete
• Embryo initially trisomic
• Trisomy rescue occurs
• Leads to:
  • CPM type 3
  • Possible uniparental disomy (UPD)

5. CPM, trisomy rescue, and UPD

When a trisomic embryo "rescues" itself:

• One chromosome is lost
• If both remaining chromosomes come from one parent → UPD

Risk depends on chromosome involved:

Chromosome	Key risks
6	Transient neonatal diabetes
7	Silver–Russell syndrome
11	Beckwith–Wiedemann
14	Temple / Kagami–Ogata
15	Prader–Willi / Angelman
16	Severe placental insufficiency

6. CPM and adverse outcomes: actual associations

Fetal Growth Restriction (FGR)

• Seen in:
  • ~10–15% of CPM type 1
  • ~20–30% of CPM type 2
  • 30–60% of CPM type 3
• Especially common with:
  • Trisomy 16 CPM
  • Trisomy 22 CPM

IUFD

• Risk increased mainly in:
  • CPM type 3
  • Large placental involvement
• Often late second or third trimester
• Placental pathology shows:
  • Infarction
  • Villous dysmaturity
  • Reduced vascularization

7. How to recognize CPM clinically (red flags)

• Positive NIPT + normal amniocentesis
• Unexplained early-onset FGR
• Discordant CVS cultures
• Placental thickening or cysts
• Abnormal uterine artery Dopplers
• Normal fetus, "bad placenta"