At what gestational age are the fetal pole and yolk sac typically visible on ultrasound?

Fetal Pole and Yolk Sac Development Timeline on Ultrasound

The yolk sac is typically visible on transvaginal ultrasound at approximately 5½ weeks gestational age, while the embryo (formerly called "fetal pole") becomes visible at approximately 6 weeks gestational age. 1

Developmental Timeline of Early Pregnancy Structures

Gestational Sac

• First visible ultrasound finding of pregnancy
• Appears at approximately 5 weeks gestational age
• Appears as a round or oval fluid collection surrounded by hyperechoic rim of trophoblastic tissue
• Without a yolk sac or embryo, should be reported as a "probable gestational sac" or "probable pregnancy"

Yolk Sac

• Appears at approximately 5½ weeks gestational age
• Appears as a thin-rimmed circular structure eccentrically located within the gestational sac
• Typically measures less than 6 mm
• Confirms definite intrauterine pregnancy when visualized
• Always located outside the amniotic cavity

Embryo

• Appears at approximately 6 weeks gestational age
• Initially appears as a thickened, linear echogenic structure at the edge of the yolk sac
• Cardiac activity is typically detectable at the time the embryo becomes visible
• The term "fetal pole" is now considered obsolete and should be avoided 1
• Crown-rump length measurement should be used for dating once the embryo is visible

Amnion

• Thin membrane surrounding the embryo
• Becomes visible at approximately 7 weeks gestational age
• Creates the amniotic cavity (fluid-filled space contained by amnion)

Clinical Implications

Diagnostic Considerations

• The sequential appearance of these structures helps in accurately dating early pregnancy 2
• Absence of expected structures by certain gestational ages may indicate abnormal development
• An empty gestational sac measuring ≥25 mm without an embryo is diagnostic of anembryonic pregnancy 3
• For embryos <7 mm without cardiac activity, follow-up ultrasound in 7-10 days is required to confirm viability 3

Terminology Notes

• The term "embryo" should be used until 10 weeks 6 days gestational age
• Beginning at 11 weeks 0 days gestational age, the term "fetus" should be used 1
• Terms like "blighted ovum" should be avoided in favor of "anembryonic pregnancy" 3
• Use "cardiac activity" rather than terms containing "heart" (e.g., "heartbeat") 1

Common Pitfalls to Avoid

1. Misinterpreting fluid in the endometrial cavity as a gestational sac

   • Fluid with pointed/non-curved margins should be described as "intracavitary fluid"
   • Avoid terms like "pseudosac" or "pseudogestational sac"

2. Making premature diagnoses

   • Don't determine pregnancy viability based on a single ultrasound too early in gestation
   • Serial measurements and appropriate timing of ultrasound are essential

3. Incorrect dating

   • Before embryo is visible, use mean sac diameter for dating
   • Once embryo is visible, crown-rump length provides more accurate dating

4. Misdiagnosing early pregnancy loss

   • Follow established criteria for diagnosing anembryonic pregnancy
   • Allow appropriate time intervals between scans when findings are inconclusive

By understanding the normal developmental timeline of early pregnancy structures, clinicians can more accurately diagnose and manage early pregnancy complications, potentially reducing unnecessary interventions and improving patient outcomes.