At how many weeks of gestation can a pregnancy be visualized on transvaginal sonography (sono)?

Visualization of Pregnancy on Transvaginal Ultrasound

A pregnancy can first be visualized on transvaginal sonography at approximately 5 weeks gestational age, when the gestational sac becomes visible. 1, 2

Timeline of Early Pregnancy Visualization

Week-by-Week Appearance of Structures

At 5 weeks gestational age:

• The gestational sac is the earliest sonographic finding, appearing as a small spherical fluid collection with a hyperechoic rim 1, 2
• Gestational sacs as small as 2-3 mm in mean sac diameter can be detected with high-frequency transvaginal transducers 1
• Some studies report visualization as early as 4.5 weeks menstrual age with endovaginal sonography 3

At 5½ weeks gestational age:

• The yolk sac becomes visible and is the first structure that definitively confirms an intrauterine pregnancy 1, 2
• The presence of a yolk sac provides incontrovertible evidence of pregnancy and should be interpreted as a "definite gestational sac" or "definite pregnancy" 1, 2

At 6 weeks gestational age:

• An embryo with cardiac activity is typically visualized 1, 2
• The embryo is identified early in the 6th week, with cardiac activity routinely identified when crown-rump length reaches 3-5 mm 3
• Cardiac activity should be documented using M-mode or videoclip and reported as "cardiac activity" or "cardiac motion" 1

At 7 weeks gestational age:

• The amniotic sac becomes visible 1, 2
• The single unpartitioned ventricle in the brain can be seen 4

Important Clinical Context

Why Earlier Visualization Is Not Possible

At 2 weeks gestational age, pregnancy cannot be visualized because it does not yet exist:

• Gestational age is calculated from the first day of the last menstrual period, not from conception 1
• At 2 weeks gestational age, ovulation is just occurring and conception has not yet happened 1
• Implantation occurs approximately 6-10 days after conception, corresponding to roughly 3-4 weeks gestational age 1
• Any ultrasound at 2 weeks will show only normal endometrium 1

Optimal Timing for Clinical Use

The earliest clinically useful transvaginal ultrasound for pregnancy confirmation should be performed at approximately 5 weeks gestational age or later 1, 2

Correlation with hCG Levels

• A gestational sac should be visible when hCG reaches approximately 1000-2000 mIU/mL 1
• Levels up to 3000 mIU/mL may be needed to reliably exclude normal intrauterine pregnancy 1

Critical Diagnostic Criteria

Confirming Viability

Presence of expected structures indicates normal pregnancy:

• Yolk sac visible in gestational sac confirms intrauterine pregnancy 1, 2
• Cardiac activity at 6 weeks with embryo crown-rump length of 3-5 mm indicates viability 3, 5

Diagnosing Nonviable Pregnancy

Absence of expected structures may indicate poor prognosis:

• Absence of yolk sac in a gestational sac >8 mm suggests nonviability 2, 3
• Absence of embryo in a gestational sac >16 mm suggests nonviability 2, 3
• Definitive diagnosis of embryonic demise requires absence of cardiac activity in an embryo ≥7 mm in crown-rump length 1, 2
• For embryos with crown-rump length <7 mm without cardiac activity, follow-up ultrasound in 7-10 days is recommended 1, 2

Common Pitfalls to Avoid

• Avoid using terms like "pseudosac" or "pseudogestational sac" as they may lead to clinical errors; instead describe fluid in the endometrial cavity based on its characteristics 2, 6
• Before visualization of a yolk sac or embryo, report an intrauterine fluid collection as a "probable gestational sac" or "probable pregnancy" 2
• Do not confuse decidual cysts with gestational sacs; decidual cysts typically lack an echogenic rim 6