By around what week of gestation can we determine chorionicity and amnionicity of a twin pregnancy by looking at the number of sacs and yokes using ultrasound imaging?

Determining Chorionicity and Amnionicity in Twin Pregnancy

Chorionicity should be determined by counting the number of gestational sacs in the first trimester, ideally as early as possible, with the intertwin membrane typically identifiable by 10 weeks on transvaginal ultrasound to confirm amnionicity. 1

Timing for Determination

Chorionicity Assessment

• Chorionicity can be determined from the time gestational sacs are first visualized in the uterus (approximately 5 weeks gestational age), with nearly 100% accuracy in the first trimester 1, 2, 3
• The optimal window is between 11-14 weeks, when chorionicity can be correctly assigned by ultrasound with 99.8% accuracy 1
• After 10 weeks, additional sonographic features become helpful including the number of placentas, lambda/twin peak sign (dichorionic) versus T-sign (monochorionic), and dividing membrane thickness 1, 2

Amnionicity Assessment

• The intertwin membrane is typically identified by 10 weeks on transvaginal ultrasound 1
• Transvaginal sonography can reliably determine amnionicity of monochorionic twins after 7 weeks menstrual age 3
• 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 1

Interpreting Number of Gestational Sacs (Chorionicity)

Dichorionic Twins

• Two separate gestational sacs, each with surrounding echogenic chorionic reactions, indicate dichorionic-diamniotic gestation 1, 4
• This is the most reliable indicator of chorionicity 2, 3

Monochorionic Twins

• A single gestational sac with peripheral chorionic reaction indicates monochorionic gestation 1, 4
• These pregnancies require more intensive surveillance due to unique complications like twin-to-twin transfusion syndrome 5

Interpreting Number of Yolk Sacs (Amnionicity)

Critical Limitation

The number of yolk sacs is NOT a reliable indicator for determining amnionicity and should not be used as the sole criterion 1, 2

Why Yolk Sac Counting is Unreliable

• Monoamniotic twin pregnancies may have two yolk sacs, contradicting the assumption that one yolk sac equals monoamniotic twins 2, 6
• Monochorionic-diamniotic pregnancies may have only one visible yolk sac early in gestation, even though they are diamniotic 2, 7
• In one study of monochorionic-diamniotic twins scanned before 8 weeks, only yolk sacs were identified initially, but the dividing amniotic membrane was not yet detectable 7

Proper Method for Amnionicity

• Demonstration of a thin intertwin membrane via transvaginal ultrasound (after 10 weeks) is the preferred method 1, 2
• If uncertainty persists, umbilical cord entanglement can be demonstrated using color or pulsed wave Doppler to identify two different heart rates, confirming monoamnionicity 2, 6

Clinical Algorithm

First Trimester Approach (Before 10 Weeks)

1. Count gestational sacs to determine chorionicity 1, 2, 3
2. Document yolk sacs but do not rely on their number for amnionicity 1, 2
3. Attempt to visualize intertwin membrane, but recognize it may not be visible yet 1
4. Schedule follow-up ultrasound after 10 weeks if amnionicity uncertain 1, 7

After 10 Weeks

1. Use transvaginal ultrasound to definitively identify intertwin membrane 1, 4
2. Assess lambda sign (dichorionic) versus T-sign (monochorionic) 1, 2
3. Count placentas and evaluate membrane thickness 1, 2
4. Use a combination of features rather than a single parameter for highest accuracy 1, 2

Critical Pitfalls to Avoid

• Never assume absence of an intertwin membrane indicates monoamnionicity before 10 weeks, as the amnion may simply not be visible yet 1, 2, 4
• Never rely solely on yolk sac number to determine amnionicity, as this has been proven unreliable 1, 2, 6, 7
• Never use a single sonographic feature alone; always combine multiple parameters for chorionicity determination 1, 2
• Failing to determine chorionicity early leads to missed opportunities for appropriate risk stratification and surveillance 2, 5

Management Implications

• All suspected twin pregnancies should have first trimester ultrasound performed, with every attempt made to determine and report chorionicity and amnionicity 8
• Monochorionic-monoamniotic twins carry significantly higher risks and should be referred to a tertiary center for specialized care 2, 5
• Transvaginal ultrasound has the highest accuracy for early assessment and should be used when transabdominal imaging is inconclusive 1, 2