What gestational age allows reliable determination of chorionicity and amnionicity on ultrasound in a multiple pregnancy?

Optimal Timing for Chorionicity and Amnionicity Determination

Chorionicity and amnionicity should be determined at the earliest imaging encounter in the first trimester, ideally between 6-10 weeks for gestational sac counting and definitively by 10-14 weeks using the lambda and T signs, as this achieves nearly 100% accuracy. 1

First Trimester Assessment (Optimal Window)

6-10 Weeks Gestation

• Counting gestational sacs at 6-10 weeks accurately reflects the number of chorions with approximately 100% sensitivity. 2
• Each gestational sac with its surrounding echogenic chorionic reaction indicates a separate chorion (dichorionic gestation). 1
• A single gestational sac with peripheral chorionic reaction indicates monochorionic gestation. 1
• Transvaginal ultrasound provides the highest accuracy for this early assessment. 1

10-14 Weeks Gestation (Peak Accuracy Period)

• The intertwin membrane is typically identified by 10 weeks on transvaginal ultrasound, making amnionicity determination reliable after this point. 1
• The lambda (twin peak) sign at 11-14 weeks indicates dichorionic placentation with 97-99% sensitivity and 95-100% specificity. 1, 3
• The T-sign is the most specific marker for monochorionicity at the intertwin membrane-placental junction. 2
• At 11-14 weeks, chorionicity was correctly assigned by ultrasound in 612 of 613 pregnancies (99.8% accuracy). 1
• Chorionicity determination between 7-12 weeks using the lambda sign led to correct assignment in all cases in one study. 4

Amnionicity Determination Timing

• Very early in pregnancy, the amnion may not yet have formed or be visible, so lack of an intertwin membrane does not necessarily indicate monoamnionicity before 10 weeks. 1
• Determination of amnionicity is thought to be less accurate before 10 weeks gestation because of delayed sonographic appearance of the thin diamniotic membrane. 1
• Reassessment for the presence of an intervening membrane should be undertaken at a later ultrasound (after 10 weeks) to confirm amnionicity. 1
• The number of yolk sacs can be erroneous for assessing amnionicity and should not be relied upon. 1

Second Trimester Assessment (Less Reliable)

• Membrane thinning, loss of the lambda sign, and apparent placental fusion reduce the reliability of chorionicity determination in the second trimester. 2
• The lambda sign tends to disappear with advancing gestational age due to regression of the chorion frondosum, becoming less reliable beyond the first trimester. 1
• High-frequency ultrasound counting membrane layers in the second trimester achieved 100% accuracy, though first trimester remains preferred. 5
• When first trimester documentation is unavailable, a combination of features must be used: membrane insertion pattern, placental number, fetal gender, and membrane thickness. 2, 6

Critical Clinical Implications

Why Timing Matters

• Monochorionic twins require intensive ultrasound surveillance every 2 weeks beginning at 16 weeks to detect twin-to-twin transfusion syndrome, selective growth restriction, and twin anemia-polycythemia sequence. 1, 2
• Dichorionic twins are monitored every 3-4 weeks without intensive placental-sharing surveillance. 2
• Monochorionic twins have approximately two-fold higher perinatal mortality (7.6% vs 1.6%) compared to dichorionic twins. 2

Consequences of Delayed or Inaccurate Determination

• Mislabeling monochorionic twins as dichorionic may under-detect serious complications requiring early intervention. 2
• Mislabeling dichorionic twins as monochorionic leads to unnecessary intensive surveillance and increased parental anxiety. 2
• Crown-rump length discordancy ≥10% in the first trimester is associated with adverse outcomes including preterm delivery, birth weight discordance, and pregnancy loss. 1

Common Pitfalls to Avoid

• Do not rely solely on placental number for chorionicity determination, especially after the first trimester, as adjacent dichorionic placentas may appear fused. 2
• Do not assume absence of an intertwin membrane indicates monoamnionicity before 10 weeks gestation. 1
• Do not use yolk sac number as the primary indicator of amnionicity, as this can be erroneous. 1
• Do not use a single sonographic feature; always combine findings such as gestational sac count, membrane characteristics, lambda/T signs, and placental appearance. 2, 6
• When uncertainty persists, implement the monochorionic surveillance protocol as the safer approach until proven otherwise. 2