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"