Is NIPT or double marker better for screening DCDA twins
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NIPT vs. Double Marker for DCDA Twin Screening
Bottom line: NIPT is significantly superior to the double marker (first-trimester combined screen) for aneuploidy screening in DCDA (dichorionic-diamniotic) twins, though each has specific caveats.
Why the Double Marker Performs Poorly in Twins
The double marker test (NT + free β-hCG + PAPP-A = first-trimester combined screen) faces fundamental limitations in twin pregnancies:
- Serum analytes (free β-hCG and PAPP-A) are not fetus-specific - they reflect the combined contribution of both placentas. This makes it impossible to attribute an abnormal result to one specific fetus.
- β-hCG and PAPP-A levels are approximately twice as high in twin pregnancies compared to singletons, complicating interpretation.
- In DCDA twins specifically, levels are significantly higher than in monochorionic twins, so chorionicity must be factored in.
- Because the distribution of serum markers in Down syndrome twin pregnancies is unknown, only a pseudorisk is calculated - designed to match singleton false-positive rates rather than maximize sensitivity.
- Data are sparse due to the low absolute number of affected twin pregnancies in published studies.
- The detection rate for T21 using NT + serum markers in twins ranges from 69%-88% at a ~5-7% false-positive rate in various studies. Second-trimester triple/quad screening performs even worse in twins (43%-63% for DCDA specifically).
Creasy & Resnik's Maternal-Fetal Medicine, p. 685-688
Why NIPT Performs Much Better in DCDA Twins
The 2021 Khalil meta-analysis (12 prospective studies, 1003 twin pregnancies) showed:
| Aneuploidy | Pooled Detection Rate | False-Positive Rate |
|---|---|---|
| Trisomy 21 | 95% (95% CI 90%-99%) | 0.09% |
| Trisomy 18 | 82% (95% CI 66%-93%) | 0.08% |
| Trisomy 13 | 80% (small numbers) | 0.13% |
The authors concluded that cfDNA (NIPT) is the most accurate screening test for T21 in twin pregnancies, with performance similar to that in singletons.
Both the ACOG/SMFM (2020 Practice Bulletin) and Harrison's Principles (2025) confirm that NIPT can be performed in twin gestations and that aneuploidy detection rates mirror those in singletons.
Creasy & Resnik's Maternal-Fetal Medicine, p. 688; Harrison's Principles of Internal Medicine 22E, p. 4032
Key Advantage of NIPT in DCDA Twins: SNP-Based Zygosity
In DCDA (predominantly dizygotic) twins, SNP-based NIPT platforms can:
- Determine zygosity (confirming if the twins are identical or fraternal)
- Measure individual fetal fractions for each twin separately
- In dizygotic twins, the average individual fetal fraction is ~6.4% (vs. 9.5% in singletons) - still usually adequate
This is clinically relevant because DCDA twins are mostly dizygotic, meaning the two fetuses may carry discordant aneuploidies. SNP-based NIPT can detect aneuploidy in one twin even when the other is euploid.
The Main Limitation of NIPT in DCDA Twins: Higher No-Call Rate
DCDA twins have a higher test failure ("no-call") rate than singletons or monochorionic twins:
- In dizygotic twins, 5.6% of cases are uninterpretable vs. 1.7% in singletons and 0.8% in monozygotic twins
- Each fetus requires a minimum individual fetal fraction of ~4% for reliable interpretation
- Predictors of failure in DCDA twins include: higher maternal weight, earlier gestational age, mode of conception (especially IVF - due to smaller placental mass), and lower PAPP-A/β-hCG
A repeat sample resolves most initial failures, but women with high pretest failure probability should be counseled about the possibility of needing invasive testing.
Practical Recommendation
| Feature | Double Marker (Combined Screen) | NIPT (cfDNA) |
|---|---|---|
| T21 detection in DCDA | ~69-88% | ~95% |
| False-positive rate | ~5-7% (pseudorisk design) | ~0.09% |
| Fetus-specific result | No | Yes (with SNP-based platforms) |
| Handles chorionicity | Requires adjustment | Provides zygosity info |
| No-call risk | None | ~5.6% in DCDA |
| Covers T13/T18 | Partially | Yes (less data) |
NIPT is the preferred first-line screening tool for DCDA twins. The double marker/NT measurement still has a role - NT in particular provides fetus-specific data and can identify structural anomalies and guide invasive testing decisions. In settings where NIPT is not available or fails (no-call result), first-trimester combined screening with NT + serum markers remains acceptable but inferior.
Note on NT (ultrasound component): In DCDA twins, NT should be measured separately for each fetus with fetus-specific risk calculation using singleton median NT reference values. This stands regardless of whether NIPT is also performed.
Sources: Creasy & Resnik's Maternal-Fetal Medicine (9780323828499); Harrison's Principles of Internal Medicine 22E (9781265977061)
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