Can Multiple Gestations Be Present with Only One Yolk Sac?
Yes, multiple gestations (specifically monochorionic twins) are absolutely possible even when only one yolk sac is visualized on early ultrasound, and yolk sac number should NOT be used as a reliable indicator of amnionicity or to rule out twins. 1, 2
Why Yolk Sac Number Is Unreliable
The most recent ACR guidelines (2024) explicitly state that "although it has been suggested that the number of yolk sacs can be used as an indicator for assessing amnionicity, this determination can sometimes be erroneous." 1 This is a critical pitfall that can lead to misdiagnosis.
Key Clinical Scenarios:
- Monochorionic-monoamniotic (MCMA) twins can present with TWO yolk sacs in up to 32% of cases, despite being in a single amniotic cavity 3
- Monochorionic-diamniotic twins may show only ONE visible yolk sac early in gestation, even though two embryos are present 2, 4
- Early monochorionic twin pregnancies appear as a SINGLE gestational sac before embryos are visualized, making it impossible to determine if multiples are present based on sac count alone 1
The Correct Approach to Determining Multiple Gestations
Before 10 Weeks:
- Visualization of embryos is REQUIRED to definitively determine singleton versus twin pregnancy 1
- A single gestational sac should be reported as "single gestational sac" rather than "single pregnancy" until embryos are seen 1
- The number of gestational sacs (chorions) determines chorionicity with nearly 100% accuracy in the first trimester, NOT yolk sac number 2
After 10 Weeks:
- The intertwin membrane should be demonstrated via transvaginal ultrasound to confirm diamniotic pregnancy 1, 2
- Additional features include: number of placentas, lambda/twin peak sign (dichorionic) versus T-sign (monochorionic), and membrane thickness 1, 2
Critical Management Implications
If only one yolk sac is seen with two embryonic poles, do NOT assume monoamnionicity. 2, 5 This requires:
- Follow-up ultrasound to definitively assign amnionicity by demonstrating the intertwin membrane 1, 2
- If monoamnionicity remains uncertain, umbilical cord entanglement can be demonstrated using color or pulsed wave Doppler showing two different heart rates 2
- MCMA twins carry significantly higher risks and require referral to a tertiary center 2, 6
Common Pitfalls to Avoid
- Never use yolk sac number alone to determine amnionicity or rule out twins 1, 2
- Do not assume absence of an intertwin membrane indicates monoamnionicity before 10 weeks, as the amnion may not yet be visible 1
- Always attempt to visualize embryos before concluding a pregnancy is singleton 1
- Research has documented multiple cases where two yolk sacs were present in confirmed MCMA pregnancies 4, 7, 3