What can be visualized on a transvaginal ultrasound at 5 weeks of gestation?

Transvaginal Ultrasound Findings at 5 Weeks of Gestation

At 5 weeks of gestation, a transvaginal ultrasound typically reveals a gestational sac as a round or oval fluid collection surrounded by a hyperechoic rim of trophoblastic tissue, with a mean sac diameter of approximately 2-3 mm. 1

Key Structures Visible at 5 Weeks

Gestational Sac

• First visible evidence of an intrauterine pregnancy
• Appears as a small spherical fluid collection with a hyperechoic rim
• Located within the endometrium, typically in the upper 2/3 of the uterus
• Mean sac diameter (MSD) of 2-3 mm, calculated as (Length + Width + Height)/3 2, 1
• Can be visualized from approximately 4.5-5 weeks of gestation 2

Supporting Signs (Not Required)

• Intradecidual sign: intrauterine fluid collection with hyperechoic rim located in the endometrium separate from the central echogenic line 2
• Double decidual sac sign: two echogenic rings around the intrauterine fluid collection 2
• Both signs have poor interobserver agreement and are not required for diagnosis of an intrauterine pregnancy 2

Yolk Sac

• May be visible at the very end of 5 weeks (approximately 5½ weeks) 1
• Appears as a thin-walled, spherical structure with an anechoic center
• Eccentrically located within the gestational sac
• First sonographic feature that definitively confirms an intrauterine pregnancy 2
• Usually visualized when gestational sac is >8 mm in mean diameter 2

What Is NOT Yet Visible at 5 Weeks

• Embryo/embryonic pole (appears at approximately 6 weeks) 1
• Cardiac activity (appears at approximately 6 weeks) 1, 3
• Amnion (appears at approximately 7 weeks) 1
• Fetal movements (appear after 7-8 weeks) 3

Clinical Considerations

Diagnostic Pitfalls

• Intracavitary fluid may mimic a gestational sac (historically called "pseudogestational sac")
• This can be distinguished by its shape (pointed or non-curved margins), contents (internal echoes), or location (in endometrial cavity) 2
• The Society of Radiologists in Ultrasound recommends avoiding terms like "pseudosac" as they may lead to clinical errors 2

Dating Considerations

• Before visualization of an embryo, the mean sac diameter can be used to estimate gestational age 1
• A gestational sac should be reported as such rather than as a "single pregnancy," as early monochorionic twin pregnancies may appear as a single gestational sac 1

hCG Correlation

• A gestational sac may not be visualized when serum hCG levels are below 1100 IU/L 4
• The discriminatory level of hCG (level at which a gestational sac should be seen) is approximately 3000 mIU/mL 2
• If no gestational sac is seen with hCG ≥3000 mIU/mL, a viable intrauterine pregnancy is unlikely 2

Follow-up Recommendations

• If findings are equivocal at 5 weeks, a follow-up ultrasound in 7-14 days is recommended 1
• Serial ultrasounds may be necessary to confirm normal early pregnancy development

Understanding what can be visualized at 5 weeks helps establish appropriate expectations and avoid misdiagnosis of early pregnancy complications.