Multiple Gestation and High-Risk Findings

Quick Answer: Set chorionicity in 1st trimester: lambda = dichorionic, T-sign = monochorionic. MC twins need serial Doppler for TTTS. >20% discordant growth is concerning. Never assume two sacs = dichorionic without lambda.

Chorionicity: The Foundation of Twin Management

Twin pregnancy ultrasound errors cause the highest-stakes registry and clinical mistakes. Chorionicity determines surveillance intensity—not "two babies visible."

After first trimester, lambda may disappear—if not documented early, late studies may be inconclusive. Registry trap: two separate placental masses do not prove dichorionicity.

Worked Scenario: 20 Weeks, Thin Membrane, No Lambda

Assume monochorionic until proven otherwise; intensify surveillance for TTTS and selective growth restriction even if early studies suggested two placentas.

Complications by Chorionicity

DCDA Twins

Lower complication rate than monochorionic; still monitor discordant growth, anomalies, and presentation. Each fetus needs complete anatomy survey and biometry.

MCDA Twins

Shared placental circulation risks:

Quintero staging (TTTS recognition): Stage I poly-oli; II absent donor bladder; III abnormal Doppler; IV hydrops; V demise.

Growth Discordance

EFW discordance = (larger EFW − smaller EFW) / larger EFW × 100%

>20–25% discordance triggers enhanced surveillance. Assign GA from earliest CRL (often larger twin) and label fetuses A/B consistently on images and reports.

Amniotic Fluid in Twins

Classic TTTS: recipient twin polyhydramnios (DVP often >8 cm), donor twin oligohydramnios (DVP <2 cm) with empty bladder in advanced stages.

Monoamniotic Twins

Single amniotic sac—cord entanglement risk; highest surveillance frequency. Document cord loops; not preventable by ultrasound alone but recognition is examinable.

Conjoined Twins

No separating membrane between fetuses; shared skin or organs. Thoracopagus (anterior chest fusion) common in registry recognition images. Diagnosed early by inability to separate twin masses.

High-Risk Maternal Conditions

NT and Aneuploidy in Twins

Measure NT in each fetus separately. Discordant NT may suggest selective aneuploidy—chorionicity determines management options (including selective reduction in specialized settings—recognition only).

Exam Traps

• Two yolk sacs early misread as late chorionicity proof
• Ignoring MCA-PSV in TAPS vignette (normal fluid volumes)
• Single NT measurement in twins
• Assuming DCDA without lambda documentation
• Missing conjoined twins by not sweeping membrane plane

Multiples multiply protocol complexity—first-trimester chorionicity commitment is the highest-yield twin topic on RDMS OB/GYN.

Twin Reversed Arterial Perfusion (TRAP)

TRAP sequence in monochorionic pregnancy shows pump twin with normal cardiac activity perfusing acardiac mass via arterial flow reversal. Acardiac twin may appear as amorphous tissue with edema. Polyhydramnios in pump twin may occur. Recognition prevents mislabeling acardiac mass as vanishing twin or teratoma.

Twin Anemia-Polycythemia Sequence (TAPS)

TAPS occurs in monochorionic twins with MCA-PSV discordance (donor anemia, recipient polycythemia) without the fluid discordance of TTTS. Registry trap: normal amniotic fluid volumes do not exclude TAPS—MCA Doppler is required in surveillance protocols for MCDA twins.

Dichorionic Twin Independence

Each dichorionic twin has separate placenta (or fused dichorionic placentas with lambda sign). Complications include discordant anomalies, one twin demise (other twin risk lower than monochorionic), and growth discordance. Complete anatomy survey and biometry per fetus—never assume concordance.

Maternal Hypertensive Disease Surveillance

Chronic hypertension and preeclampsia increase IUGR, oligohydramnios, and abnormal UA Doppler risk. Surveillance includes serial growth, BPP, umbilical artery Doppler, and middle cerebral artery when indicated. Cerebroplacental ratio integrates MCA and UA for placental insufficiency assessment.

Diabetes and Fetal Effects

Preexisting and gestational diabetes increase macrosomia, cardiac anomaly risk, and polyhydramnios. Target EFW and AC percentiles, not single glucose value. Shoulder dystocia risk rises with EFW >4000 g—sonographic estimate guides counseling, not cesarean decision alone.

Rh Isoimmunization and Fetal Anemia

In alloimmunized pregnancy, MCA peak systolic velocity screens for fetal anemia—elevated PSV prompts cordocentesis or intrauterine transfusion in specialized centers. Document PSV multiples of median (MoM) per protocol. This overlaps Doppler domain but appears in high-risk OB vignettes.

Prior Cesarean and Placenta Accreta Risk

Anterior placenta previa with prior cesarean is highest-risk accreta scenario. Ultrasound markers include loss of clear zone, lacunae, bladder wall interruption, and subplacental hypervascularity. MRI may follow positive sonographic suspicion—sonographer documents findings, does not stage surgery.

TTTS and MCA Doppler

TTTS shows polyhydramnios recipient and oligohydramnios donor in monochorionic twins. TAPS shows MCA-PSV discordance without fluid discordance—do not rely on fluid alone in MCDA surveillance.

Chorionicity Documentation

Lambda sign confirms dichorionic; T-sign confirms monochorionic. Document in first trimester—late studies may be inconclusive. EFW discordance >20–25% triggers enhanced surveillance.

Twin Labeling Discipline

Label twins A/B consistently on images and reports. Conjoined twins show no separating membrane and shared skin or organs—diagnose by inability to separate fetal masses on thorough sweep.