What can be visualized on a 6th week pregnancy scan, including the gestational sac, yolk sac, and embryo?

What Can Be Visualized on a 6th Week Pregnancy Scan

At 6 weeks gestational age, a transvaginal ultrasound should show a gestational sac, yolk sac, and embryo with cardiac activity. 1, 2

Key Structures Visible at 6 Weeks

Gestational Sac

• First sonographic finding of pregnancy, typically visualized from 5 weeks gestational age 1
• Appears as a round or oval fluid collection surrounded by a hyperechoic rim of trophoblastic tissue 1
• Located within the upper 2/3 of the uterus in normal pregnancies 2
• Mean sac diameter (MSD) can be calculated using the formula: (Length + Width + Height)/3 1

Yolk Sac

• Thin-rimmed circular structure eccentrically located within the gestational sac 1
• First structure visible within the gestational sac, confirming a definite intrauterine pregnancy 1, 2
• Typically visualized from approximately 5½ weeks gestational age 1
• Usually measures less than 6 mm in diameter 1
• Always located outside the amniotic cavity 1

Embryo

• Visible by transvaginal ultrasound at 6 weeks gestational age 1, 2
• Cardiac activity should be present and visible at 6 weeks 1, 2
• Cardiac activity should be documented using M-mode or cine clip 1, 2
• Crown-rump length (CRL) measurement is the most accurate for dating once the embryo is present 2
• Embryonic cardiac activity is typically present when the embryo measures 3-5 mm in crown-rump length 3

Structures Not Yet Visible at 6 Weeks

• Amnion: typically visualized at approximately 7 weeks gestational age 1, 2
• Embryonic body movements: typically observed after 7-8 weeks gestational age 4

Clinical Significance and Potential Pitfalls

Confirmation of Viable Intrauterine Pregnancy

• The presence of a yolk sac within an intrauterine fluid collection provides incontrovertible evidence of a definite intrauterine pregnancy 1
• Absence of cardiac activity in an embryo with CRL >3-5 mm indicates embryonic demise 3

Potential Pitfalls

• Misinterpreting intracavitary fluid as a gestational sac can lead to incorrect diagnosis 1, 2
• Missing multiple gestations can have significant implications for pregnancy management 2
• Before visualization of an embryo, a single 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

Warning Signs

• Absence of a yolk sac when the mean gestational sac diameter exceeds 8 mm may indicate a nonviable pregnancy 3
• Absence of an embryo when the mean gestational sac diameter exceeds 16 mm may indicate a nonviable pregnancy 3
• Deformed or abnormally sized yolk sac may predict pregnancy loss 5

Technical Considerations

• Transvaginal ultrasound is superior for visualizing early pregnancy structures 2
• A combined approach of both transabdominal and transvaginal ultrasound may be beneficial 2
• When findings are equivocal, a follow-up ultrasound in 7-14 days is recommended 2

Remember that visualization of structures depends on the exact gestational age and the quality of the ultrasound equipment. Transvaginal ultrasound provides better resolution for early pregnancy assessment than transabdominal ultrasound.