What happened to the fetus in a pregnancy that showed a gestational sac with a fetus one week, but only two yolk sacs and no fetus the following week, indicating a miscarriage?

Understanding the "Vanishing Twin" Phenomenon in Early Pregnancy Loss

What you're experiencing is a "vanishing twin" phenomenon, where one embryo in an early twin pregnancy has been reabsorbed, leaving behind two yolk sacs but no visible fetus on the follow-up ultrasound. 1

What Happened to the Fetus?

When a fetus is visible on ultrasound one week but not visible the following week, with only yolk sacs remaining, this indicates an early pregnancy loss (EPL) through a process called embryonic resorption. Here's what occurred:

• Initially, your ultrasound showed a gestational sac containing a fetus, suggesting a viable pregnancy
• The follow-up ultrasound revealed two yolk sacs but no fetus, indicating:
  • The pregnancy was likely a twin pregnancy (explaining the two yolk sacs)
  • The embryo(s) stopped developing and underwent resorption
  • The yolk sacs remained visible despite embryonic demise

Why Two Yolk Sacs Appeared

The appearance of two yolk sacs on your follow-up ultrasound when only one was previously seen can be explained by:

1. Twin pregnancy with one "vanishing twin": This is the most likely explanation. The American College of Radiology guidelines note that "caution is warranted in establishing viability of a twin during early pregnancy because the demise of one of the twins is relatively common, resulting in the so-called 'vanishing twin'" 1

2. Improved visualization: Sometimes structures that weren't clearly visible on an earlier scan become more apparent on follow-up imaging

Diagnostic Criteria for Early Pregnancy Loss

According to current guidelines, embryonic demise (miscarriage) is confirmed when:

• An embryo ≥7 mm crown-rump length has no cardiac activity 1, 2
• A gestational sac ≥25 mm mean sac diameter contains no embryo 1, 2
• No embryonic development is seen on follow-up ultrasound after appropriate intervals 1, 2

In your case, the absence of a previously seen embryo on follow-up ultrasound confirms that the pregnancy is non-viable.

Common Pitfalls in Early Pregnancy Ultrasound Interpretation

• Mistaking yolk sacs for embryos: The yolk sac is typically visible before the embryo and appears as a thin-walled, spherical structure with an anechoic center 1
• Relying solely on a single ultrasound: Serial ultrasounds are often needed to confirm pregnancy viability or non-viability 2
• Misinterpreting the number of yolk sacs: Although it has been suggested that a monoamniotic twin pregnancy has a single yolk sac, this may not always be the case, and determining the number of yolk sacs is not an absolutely accurate indicator of amnionicity 1

What Happens Next

The pregnancy tissue (including yolk sacs) will likely be expelled naturally in the coming days or weeks. If this doesn't occur, medical or surgical management may be necessary to complete the miscarriage process.

Key Points to Remember

• The "vanishing twin" phenomenon occurs in approximately 20-30% of twin pregnancies identified in the first trimester 1
• The absence of a previously seen embryo on follow-up ultrasound is diagnostic of pregnancy loss
• Two yolk sacs indicate the pregnancy was likely a twin pregnancy initially
• This is a common occurrence and does not necessarily indicate any underlying health issues that would affect future pregnancies

If you experience heavy bleeding, severe pain, fever, or other concerning symptoms, seek immediate medical attention as these could indicate complications of the miscarriage process.