Dual Marker (Single & Twin)

This is the same biochemistry as the standard Dual Marker — PAPP-A and free β-hCG measured at 11+0 to 13+6 weeks — but the risk algorithm is adapted for twin pregnancies. In twins, both placentas contribute hormones to the maternal circulation, so absolute serum levels are typically 1.5–2× higher than singleton values. Without twin-specific MoM correction, every twin pregnancy would falsely appear "high-risk."

For twins, the risk is calculated separately depending on chorionicity (determined by ultrasound). Monochorionic twins share one placenta and are treated as a single risk per pregnancy. Dichorionic twins (separate placentas) get a risk calculated per foetus, although the biochemistry contribution still has limits — twin screens are less accurate than singleton screens (detection ~75–80% for Down vs 85–90% in singletons).

Twin pregnancies are increasingly common in India due to assisted reproduction (IVF, IUI). They carry higher base rates of structural and chromosomal abnormalities and significantly higher pregnancy risks (preterm birth, growth discordance, pre-eclampsia, twin-to-twin transfusion syndrome in monochorionic twins). Reliable first-trimester screening is therefore even more important — and it requires a lab and algorithm that explicitly handle twin pregnancies.

Choosing the wrong "singleton" Dual Marker for a twin pregnancy will mis-estimate the risk. The "Single & Twin" version is the correct selection if there is any possibility of multiples — even before the NT scan confirms it. NIPT for twins is also feasible from 10 weeks, with caveats around lower fetal fraction.

No fasting required. Schedule the blood draw close to your NT scan (ideally same day or within 1–2 days). The NT report must state chorionicity (monochorionic vs dichorionic) — this is essential for the risk calculation. Disclose IVF/IUI conception, ethnicity, smoking, and prior pregnancies on the request form.