Twin pregnancies require careful ultrasound evaluation from the very beginning — the most important determination is chorionicity, because monochorionic twins carry significantly higher risk than dichorionic twins. Get this wrong and clinical management is compromised.

Why Chorionicity Matters

Chorionicity (whether twins share a placenta) determines risk:

Type	Shared?	Perinatal Mortality Risk
Dichorionic Diamniotic (DCDA)	Separate placentas, separate sacs	~3–4%
Monochorionic Diamniotic (MCDA)	Shared placenta, separate sacs	~15%
Monochorionic Monoamniotic (MCMA)	Shared placenta, shared sac	~30–50%

MCDA twins are at risk for twin-to-twin transfusion syndrome (TTTS), selective fetal growth restriction (sFGR), and twin anemia-polycythemia sequence (TAPS). MCMA twins have additional risk from cord entanglement.

Bottom line: Chorionicity must be established at the first trimester ultrasound when it is most accurate.

First Trimester Determination (< 14 Weeks)

The first trimester is the best time — accuracy approaches 100% for chorionicity and 98% for amnionicity.

Number of Gestational Sacs

Two sacs → Dichorionic (DI): Each embryo in its own chorionic sac. If two clearly separate sacs are seen, it's dichorionic regardless of placentation appearance.

One sac → Monochorionic (MC): One chorionic sac with one or two amnions inside.

Number of Yolk Sacs

Within a single chorionic sac:

Two yolk sacs → MCDA (two amnions — each embryo has its own amnion, but share chorion)
One yolk sac → MCMA (one amnion — both embryos share everything)

Membrane Findings

By 8–10 weeks the dividing membrane (if present) is visible as a thin line.

Second and Third Trimester Determination

When first-trimester imaging is unavailable, chorionicity is harder to determine but several signs help.

The Lambda Sign (Twin Peak Sign) — Dichorionic

Lambda sign: A wedge of placental tissue extending into the base of the inter-twin membrane — formed by chorionic villi infiltrating the membrane between the two placentas.

Appearance: A triangular echogenic tissue projection at the membrane's placental insertion — "a peaked cap" at the base.

Present in dichorionic diamniotic twins only. Sensitivity > 90% in second trimester; decreases with advancing gestation.

The T-Sign — Monochorionic

T-sign: The inter-twin membrane inserts at a right angle (90°) to the placenta forming a "T" shape. There is no placental tissue infiltrating the base of the membrane.

Present in monochorionic diamniotic twins. The membrane is thin and inserts directly onto the chorionic plate.

Summary:

Sign	Chorionicity
Lambda / twin peak	Dichorionic diamniotic
T-sign	Monochorionic diamniotic
No membrane	Monochorionic monoamniotic

Placental Number and Location

Two placentas clearly separated: Dichorionic
Single placenta with fused appearance: May be DCDA (fused) or MCDA — membrane sign is needed
Single placenta with T-sign: MCDA

Membrane Thickness

DCDA: Membrane is thick (four layers — two chorions + two amnions) MCDA: Membrane is thin (two layers — amnion only)

Membrane thickness ≥ 2 mm favors dichorionic. However, this becomes less reliable with advancing gestation.

Fetal Sex

Discordant sex = almost certainly dichorionic (unless sex chromosomal anomaly). Concordant sex: compatible with either — cannot distinguish chorionicity by sex alone.

Routine Twin Surveillance

DCDA twins:

Growth ultrasound every 4 weeks from 24 weeks
Antenatal surveillance typically begins at 36 weeks

MCDA twins (higher risk):

Serial ultrasound every 2 weeks from 16 weeks for TTTS surveillance
Doppler assessment (MCA PSV, umbilical artery, ductus venosus) when growth discordance or TTTS suspected

MCMA twins:

Most centers admit for inpatient monitoring at 26–28 weeks
Twice-weekly or more frequent biophysical profiles
Cord entanglement identified on ultrasound (intertwining)

Twin-to-Twin Transfusion Syndrome (TTTS)

TTTS occurs in ~10–15% of MCDA pregnancies. Vascular anastomoses on the shared placenta allow unbalanced blood flow from the donor twin to the recipient twin.

Quintero Staging System:

Stage	Criteria
I	Donor: oligohydramnios (DVP < 2 cm). Recipient: polyhydramnios (DVP > 8 cm). Bladder of donor visible
II	Donor bladder not visible on prolonged scanning
III	Abnormal Doppler: absent/reversed EDV in umbilical artery, reversed ductus venosus a-wave, or pulsatile umbilical vein
IV	Hydrops in either twin
V	Fetal demise of either twin

Sonographic TTTS Findings

Donor twin (the one giving blood away):

Oligohydramnios: DVP < 2 cm (single deepest pocket)
Small for gestational age, growth restricted
Bladder may be absent / very small
Umbilical artery: elevated RI, absent or reversed end-diastolic flow
Stuck twin: amniotic sac wraps tightly around the stuck donor (no fluid to move in)

Recipient twin (the one receiving blood):

Polyhydramnios: DVP > 8 cm
Larger than donor
Enlarged, full bladder (urinates into the sac — that's why it's full)
Cardiac hypertrophy, cardiomegaly from volume overload
Hydrops in advanced cases (skin edema, ascites, pleural/pericardial effusion)

Measuring Fluid

Technique: In MCDA twins, always measure the DVP (deepest vertical pocket) in each sac separately.

Identify the inter-twin membrane (thin T-sign membrane)
Measure DVP in the donor's sac (should be < 2 cm in TTTS Stage I+)
Measure DVP in the recipient's sac (> 8 cm in TTTS Stage I+)

Do not use AFI in twin pregnancies — DVP is standard for multiple gestation.

Selective Fetal Growth Restriction (sFGR)

In MCDA twins, when one twin's EFW is below the 10th percentile AND there's > 25% EFW discordance between twins. Not the same as TTTS — no fluid discordance required.

Graded by umbilical artery Doppler:

Type I: normal or reduced end-diastolic flow
Type II: absent or reversed EDV (worst prognosis)
Type III: intermittent absent/reversed EDV (most unpredictable)

What to Document in Every Twin Scan

For each twin:

Labeling: Twin A (presenting/lower twin), Twin B (upper twin)
EFW and biometry (BPD, HC, AC, FL)
EFW percentile for gestational age
Amniotic fluid (DVP in each sac)
Bladder visualization
Fetal activity and tone (biophysical profile components as appropriate)
Doppler: umbilical artery waveform (if indicated)
Chorionicity documentation at each scan

Reporting Language

Chorionicity: "Monochorionic diamniotic twin gestation confirmed. A thin inter-twin membrane with T-sign insertion into the anterior placenta is identified. Single anterior placenta. Twin A is the lower twin."

TTTS Stage II: "Twin A (donor): Oligohydramnios with DVP 1.2 cm. Bladder not identified on 30-minute observation. Umbilical artery: reversed end-diastolic flow. EFW 423 g (< 5th percentile). Twin B (recipient): Polyhydramnios with DVP 10.4 cm. Bladder markedly distended. EFW 712 g (28th percentile). EFW discordance 40%. Findings consistent with twin-to-twin transfusion syndrome, Quintero Stage III. Urgent maternal-fetal medicine consultation is recommended."

SonoBuddy's OB protocols include chorionicity determination, growth scans, and fetal Doppler guides.

Multiple Gestation Ultrasound: Twins, Chorionicity, and TTTS