The best time to identify the type of twin pregnancy on ultrasound is:

Correct Answer: B. 11-14 weeks

The 11–14 week window is the gold standard for twin chorionicity determination because this is when the intertwin membrane (lambda sign or twin peak sign) is most clearly visualized and the nuchal translucency (NT) can be measured accurately for aneuploidy screening in both fetuses. At this gestation, the placental interface is still relatively thin and echogenic, making the lambda sign (triangular projection of placental tissue into the intertwin membrane in dichorionic twins) or the T sign (thin membrane in monochorionic twins) distinctly visible. The discriminating principle: chorionicity is fixed at conception and does not change, but its sonographic visibility varies with gestational age. Before 11 weeks, the gestational sacs may be too small for reliable assessment. After 14 weeks, the intertwin membrane becomes progressively thinner and less echogenic, the placental interface flattens, and the lambda sign regresses, making differentiation increasingly difficult. Indian guidelines (FOGSI, IAP) and standard obstetric practice recommend first-trimester assessment (11–14 weeks) as part of the routine dating scan and aneuploidy screening protocol. Accurate chorionicity determination is critical for antenatal surveillance intensity, delivery planning, and counselling regarding perinatal risks (TTTS, selective IUGR, cord entanglement in monochorionic monoamniotic twins).

Why the other options are wrong

A. 24-28 weeks — By the third trimester, the intertwin membrane is thin, poorly echogenic, and the lambda sign has regressed significantly. Chorionicity assessment becomes unreliable and difficult; many monochorionic twins are misclassified as dichorionic at this stage. This is too late for accurate determination and defeats the purpose of early risk stratification. C. 14-18 weeks — Although still in the second trimester, the optimal window has passed. The lambda sign begins to fade after 14 weeks, and the membrane becomes progressively thinner. While assessment is still possible, accuracy drops compared to 11–14 weeks, and the opportunity for first-trimester aneuploidy screening is lost. D. 18-22 weeks — By this stage, the intertwin membrane is thin and difficult to visualize clearly. The lambda sign is largely regressed, and the T sign (if present) is subtle. This timing misses the critical first-trimester window and reduces diagnostic confidence, making it unsuitable for routine chorionicity determination.

High-Yield Facts

Mnemonics

LAMBDA = Dichorionic Lambda sign = Large, triangular, echogenic projection = Dichorionic (two placentas pushing into membrane). Visible 11–14 weeks only. Early = Accurate First trimester (11–14 wks) = Best visibility of membrane interface. Second trimester onward = Membrane thins, lambda fades, accuracy drops. Do it early or regret it.

NBE Trap

NBE may lure candidates into selecting second or third-trimester windows by framing the question as "best time to identify" without emphasizing that chorionicity assessment becomes progressively unreliable after 14 weeks. Students unfamiliar with the regression of the lambda sign may incorrectly assume that later scans (with better fetal biometry) are equally suitable.

Clinical Pearl

In Indian antenatal practice, the 11–14 week scan serves dual purpose: dating the pregnancy and determining chorionicity. A monochorionic twin pregnancy identified early allows the obstetrician to counsel the family about TTTS risk (~10–15%), plan fortnightly Doppler surveillance from 16 weeks, and prepare for possible in-utero intervention (selective feticide, LASER ablation) if complications arise—a window of opportunity lost if assessment is delayed.

_Reference: DC Dutta's Textbook of Obstetrics, Ch. 18 (Multiple Pregnancy); FOGSI Guidelines on Antenatal Care; Harrison Ch. 346 (Multiple Gestation)_