Diagnosing Chorionicity
in Twin Pregnancies

A stepwise sonographic algorithm for sonographers

Maternal-Fetal Medicine  ·  Ultrasound Education  ·  2026

Clinical Importance

Why Chorionicity Matters

3–5×
higher perinatal loss risk
monochorionic vs. dichorionic twins
43%
of multifetal pregnancies lack accurate
chorionicity diagnosis before tertiary care
🔁
Twin-to-Twin Transfusion Syndrome (TTTS)Vascular anastomoses in shared placenta cause hemodynamic imbalance
🫀
Twin Reversed Arterial Perfusion (TRAP)Acardiac twin perfused in reverse by co-twin's circulation
📉
Selective Fetal Growth Restriction (sFGR)Unequal placental sharing leads to discordant fetal growth
📋
Guides Clinical ManagementDetermines surveillance frequency, invasive procedure planning, and MFM referral
Fundamentals

Zygosity vs. Chorionicity

Dizygotic (DZ) Twins

  • Two ova fertilized by two sperm
  • Always dichorionic (DC)
  • ~70% of all twin pregnancies

Monozygotic (MZ) Twins

  • Single zygote divides
  • ~30% of all twin pregnancies
  • Chorionicity depends on timing of division
Corner's Theory — Timing of Zygote Division
Days Post-FertilizationResultFrequency
0 – 3 daysDCDA Dichorionic Diamniotic29%
4 – 7 daysMCDA Monochorionic Diamniotic70%
8 – 12 daysMCMA Monochorionic Monoamniotic1%
13 – 16 daysConjoined<1/200,000
💡
Key Principle

Chorionicity reflects placentation, not zygosity. Manage based on chorionicity, not assumed zygosity.

Timing

Optimal Diagnostic Window

4–5
wk
Count chorionic sacs
(transvaginal)
Reliable DC vs MC
6 wk
Count yolk sacs
+ embryonic poles
Confirm MC if single sac
8 wk
Amnion visibility
improves
Assess amnionicity
10–14
wk
Lambda / T Sign
+ Placental Mass
99.8% accuracy ★ Optimal
>14
wk
Multi-parameter
approach required
Accuracy drops ~77%
⚠️
NICE / SMFM Recommendation

Determine chorionicity at the first scan identifying twins. Document clearly and assign fetal nomenclature (e.g., upper/lower, left/right).

Before 10 Weeks

Transvaginal Approach

Two Gestational Sacs

  • Most reliable early sign
  • Confirms Dichorionic (DC)
  • Two echogenic rims in thick decidua

Single Sac + Two Embryos

  • Suggests Monochorionic (MC)
  • Two yolk sacs visible by 6 weeks
  • Amnionicity unreliable before 8 wk
⚠️
Caution: Amnionicity Before 8 Weeks

Thin diamniotic membrane may not be visible. Do not assume monoamniotic if no dividing membrane is seen — reassess at a later scan. Number of yolk sacs is not a reliable indicator of amnionicity.

4–5 WK · TRANSVAGINAL Sac 1 Sac 2 DICHORIONIC 6 WK · SINGLE SAC YS1 YS2 MONOCHORIONIC
10–14 Weeks

The Lambda (λ) Sign — Twin Peak Sign

Lambda (λ) Sign — Dichorionic

PLACENTA λ Chorionic villi fill wedge Intertwin membrane

Triangular wedge of placental tissue projects into the membrane base. Echogenicity matches placenta.

Diagnostic Accuracy (First Trimester)
MetricValue
Sensitivity99%
Specificity95%
Best window10–14 weeks
Meta-analysis n2,292 twin pairs
⚠️
"Vanishing" Lambda Sign

Lambda sign can regress in the 2nd trimester in DC twins. Absence after 14 weeks does not confirm monochorionicity.

Scanning Tip

Angle the probe to visualize the membrane insertion site en face. The lambda sign is best seen where the membrane meets the placental surface.

10–14 Weeks

The T Sign — Monochorionic Diamniotic

T Sign — Monochorionic

PLACENTA T No tissue wedge Perpendicular insertion

Membrane inserts perpendicularly into placenta. No triangular tissue projection. Thin, hairlike membrane.

Diagnostic Accuracy (First Trimester)
MetricValue
Sensitivity~100%
Specificity98%
Combined withSingle placental mass
DiagnosisMCDA
Study Accuracy — Lambda + T Sign
Author (Year)nSens %Spec %
Stenhouse 20029610098.7
Carroll 20021509897.4
Menon 200534410097.9
Lee 200624789.899.5
Dias 201161310099.8
After 14 Weeks

Late Determination — Reduced Accuracy

Why Accuracy Drops
📉
Lambda sign regressesDisappears by 20 wk in ~7% of DC twins with fused placentas
🔬
Membrane thinningProgressive thinning reduces layer-counting accuracy
🧩
Placental fusionDC fused placentas mimic single MC placental mass
Accuracy of Late-Pregnancy Parameters
ParameterAccuracy
Lambda / T sign~77%
Membrane thickness (≥2 mm)76–83%
Membrane layer count~83% (highest)
Discordant fetal sexPPV ~100%
Two separate placentasSensitivity 32%, PPV 97.7%
💡
Use All Five Parameters Together

Combining sex, placental number, lambda/T sign, membrane thickness, and membrane layers in a stepwise algorithm maximizes accuracy after 14 weeks.

Membrane Assessment

Intertwin Membrane Characteristics

MEMBRANE CROSS-SECTION DICHORIONIC 4 layers · ≥2 mm Amnion Chorion Chorion Amnion ≥2mm Thick, echogenic MONOCHORIONIC 2 layers · <2 mm Amnion Amnion <2mm Thin, hairlike

Dichorionic Membrane

  • 4 layers: Amnion–Chorion–Chorion–Amnion
  • Thickness ≥ 2 mm
  • Thick, echogenic appearance
  • Measure close to placenta (<3 cm)

Monochorionic Membrane

  • 2 layers: Amnion–Amnion only
  • Thickness < 2 mm
  • Thin, hairlike appearance
  • Sensitivity declines to 52% in 3rd trimester
🔬
Scanning Technique

Image membrane perpendicular to ultrasound beam with magnification. Measure close to placental insertion (<3 cm). Layer counting is more accurate in 2nd than 3rd trimester.

Algorithm

Stepwise Sonographic Algorithm

TWIN PREGNANCY DETECTED Assess gestational age GA < 10 wk? transvaginal YES Count chorionic sacs / yolk sacs Reassess at 10–14 wk NO GA 10–14 wk? optimal window YES Placental mass count Lambda sign → DC T sign → MCDA Accuracy ~99.8% NO (>14 wk) Multi-Parameter Approach Fetal sex · Placental number · Lambda/T sign · Membrane thickness · Membrane layers If uncertain → Manage as MONOCHORIONIC · Refer to MFM Specialist
Practical Tips

Sonographer Best Practices

📅
Document at First ScanAssign fetal nomenclature (upper/lower, left/right). Progressively harder to determine later.
🔍
Use Transvaginal US When NeededIf transabdominal imaging is inconclusive, particularly in early gestation.
📐
Angle Probe En FaceExamine membrane-placenta junction carefully. Lambda sign best seen at insertion site.
🧮
Combine Multiple SignsDo not rely on a single feature. Use placental number + lambda/T sign + membrane characteristics together.
🔁
Reassess If UncertainRe-examine within 1–2 weeks or refer to MFM specialist or tertiary center.
⚠️
Beware Vanishing LambdaLambda sign can regress in 2nd trimester even in DC twins. Absence later does not confirm MC.
🔴
Golden Rule: Default to MCWhen in doubt, manage as monochorionic until proven otherwise (TTTS, TAPS, sFGR risk).
📊
Audit & TrainingRegular clinical audits and training on lambda/T sign identification are recommended (NICE guideline).
Summary

Key Takeaways

⏱️
Optimal Window
10–14 weeks achieves 99.8% accuracy. Determine at first scan.
λ / T
Primary Signs
Lambda = DC. T sign = MCDA. Most reliable first-trimester indicators.
🔴
Golden Rule
When uncertain, manage as monochorionic. Refer to MFM if needed.

Dichorionic (DC)

  • Two gestational sacs (early)
  • Lambda (λ) / Twin Peak sign
  • Two separate placentas
  • Discordant fetal sex
  • Thick membrane ≥ 2 mm, 4 layers

Monochorionic (MC)

  • Single gestational sac (early)
  • T sign — perpendicular insertion
  • Single placental mass
  • Thin membrane < 2 mm, 2 layers
  • Higher risk: TTTS, TRAP, sFGR
← OpenMFM Library