At what gestational age can transvaginal ultrasounds (TVUS) detect twins?

Detection of Twins by Transvaginal Ultrasound

Transvaginal ultrasound can detect twin pregnancies as early as 6 weeks of gestation, with optimal visualization and accurate determination of chorionicity achieved by counting gestational sacs in the first trimester, ideally before 10-14 weeks. 1, 2

Early Detection Timeline

First Trimester Detection (6-10 Weeks)

• Twin pregnancies can be visualized as early as 6 weeks using transvaginal ultrasound, where separate gestational sacs become identifiable 3
• The American College of Radiology recommends transvaginal ultrasound as the single best diagnostic modality for evaluating early pregnancy, particularly for assessing the endometrium, early pregnancy structures, and adnexa 2
• All patients suspected of having twin pregnancy on first trimester physical examination or at risk (such as those with assisted reproductive technology pregnancies) should have first trimester ultrasound performed 4

Optimal Window for Chorionicity Determination (First Trimester)

• The most accurate determination of chorionicity is achieved in the first trimester by counting gestational sacs, with nearly 100% accuracy 1
• The number of gestational sacs equals the number of chorions, making this the gold standard approach 1
• Every attempt should be made to determine and report amnionicity and chorionicity when a twin pregnancy is identified at the earliest ultrasound evaluation 4, 1

Technical Considerations for Early Detection

Transvaginal vs. Transabdominal Approach

• A combined transabdominal and transvaginal approach should be performed when possible at 8-10 weeks, allowing comprehensive evaluation of structures that may be positioned too high for transvaginal visualization alone 2
• Transvaginal ultrasound is particularly useful as a complement to transabdominal scanning during the first trimester and up to 16 weeks of gestation 2
• Transabdominal ultrasound alone may be insufficient in early pregnancy because the gestations are still relatively small and may not be optimally visualized through the abdominal wall, particularly in patients with higher body mass index 2

Amnionicity Assessment After 10 Weeks

• For amnionicity determination, careful evaluation of the intertwin membrane via transvaginal ultrasound after 10 weeks gestation is more reliable than counting yolk sacs 1
• The number of yolk sacs is not an absolutely accurate indicator of chorionicity, as monoamniotic twin pregnancies may have two yolk sacs and monochorionic-diamniotic twin pregnancies may have only one visible yolk sac 1
• When amnionicity remains uncertain, confirmation can be achieved by demonstrating umbilical cord entanglement using color or pulsed wave Doppler to identify two different heart rates 1

Clinical Implications of Early Detection

Risk Stratification

• Accurate determination of chorionicity and amnionicity is essential for proper risk stratification and management of multiple gestations 1
• Monochorionic-monoamniotic twin pregnancies carry significantly higher risks and should be referred to a tertiary center for specialized care 1
• Failing to correctly determine chorionicity can lead to missed opportunities for early intervention in high-risk pregnancies 1

Special Circumstances

• Conjoined twins can be diagnosed as early as 9-10 weeks of gestation using transvaginal two-dimensional ultrasound and color Doppler examination 5, 6
• Three-dimensional transvaginal ultrasonography can provide accurate diagnosis of chorionicity and amnionicity at 6 weeks of pregnancy, particularly useful when two-dimensional imaging is equivocal 3

Common Pitfalls to Avoid

• Do not rely solely on transabdominal ultrasound in early pregnancy, as resolution may be inadequate for complete assessment of early pregnancy structures 2
• When chorionicity cannot be determined with certainty on transabdominal scan, a transvaginal scan should be performed 1
• Avoid using a single feature rather than a combination of sonographic findings to determine chorionicity 1
• Do not assume absence of an intertwin membrane indicates monoamnionicity, as the amnion may not yet be visible at early gestational ages 1