Is the risk of having one fetus with aneuploidy (chromosomal abnormality) highest in triamniotic trichorionic (three separate placentas and amniotic sacs) triplet pregnancies?

Risk of Aneuploidy in Triplet Pregnancies Based on Chorionicity

The risk of having one fetus with aneuploidy is highest in triamniotic monochorionic triplet pregnancies compared to other types of triplet pregnancies. 1

Explanation of Risk by Chorionicity Type

Triamniotic Monochorionic (c)

• Highest risk of aneuploidy due to:
  • Monozygotic twins have 2-3 times higher risk of congenital anomalies compared to singletons 1
  • Later splitting of monozygotic embryos increases risk of chromosomal abnormalities 1
  • Possibility of heterokaryotypic monozygotism where chromosomal abnormality occurs during early cleavage divisions 2
  • Monochorionic triplets have a 13.7% incidence of one or more fetal structural abnormalities 3

Triamniotic Dichorionic (b)

• Intermediate risk:
  • Contains a monochorionic pair with associated higher risk of anomalies 4
  • Lower risk than completely monochorionic pregnancies 5
  • Dichorionic triamniotic triplets show increased morbidity compared to trichorionic triplets 4

Triamniotic Trichorionic (a)

• Lowest risk:
  • Each fetus has separate placenta (trichorionic)
  • No shared placental circulation to complicate development
  • Typically results from separate embryos (higher proportion of dizygotic/trizygotic conception)
  • Absence of monochorionic complications that can affect development 5

Risk Factors and Complications

Monochorionic-Specific Risks

• Unequal sharing of placenta and vascular communications 1
• Twin-to-twin transfusion syndrome (TTTS) in 27.6% of monochorionic triplets 3
• Twin anemia-polycythemia sequence (TAPS) in 4.6% of cases 3
• Selective fetal growth restriction (sFGR) in 16.4% of cases 3
• Higher risk of stillbirth (5.4 times higher than trichorionic) 5

Nuchal Translucency and Screening

• Nuchal translucency measurements are typically higher in monochorionic pregnancies 1
• Nuchal translucency discordance >20% associated with >30% risk of severe TTTS 1
• First trimester combined screening in monochorionic twins has higher false-positive rate 1

Clinical Implications

• Early determination of chorionicity is crucial for risk assessment and management
• Fetal echocardiography is warranted in all monochorionic triplets regardless of nuchal translucency measurements 1
• Monochorionic triplets require more intensive surveillance (every 2 weeks starting at 16 weeks) 1
• Growth discordance is greater in pregnancies with monochorionic component 5

Pitfalls and Caveats

• Chorionicity must be determined in the first trimester when signs are most reliable 1
• Monochorionic triplets can have discordant anomalies despite being monozygotic 1
• Aneuploidy can occur in just one fetus of a monochorionic set due to post-zygotic mitotic errors 2
• Do not assume all fetuses in a monochorionic pregnancy will have identical chromosomal status

In summary, the risk gradient for aneuploidy in triplet pregnancies is: monochorionic triamniotic (highest) > dichorionic triamniotic > trichorionic triamniotic (lowest).