Intrauterine Pregnancy of Uncertain Viability
The most likely diagnosis is D. Intrauterine pregnancy of uncertain viability. At 5 weeks gestational age with an intrauterine gestational sac but no embryonic heartbeat visible, this pregnancy cannot yet be definitively classified as viable or nonviable, and requires follow-up imaging to establish the diagnosis 1.
Diagnostic Reasoning
Why this is an intrauterine pregnancy of uncertain viability:
- At 5 weeks gestation (corresponding to approximately 4.5-5 weeks from conception), a gestational sac measuring 2-3 mm in mean sac diameter may be visualized on transvaginal ultrasound 1.
- The absence of an embryonic heartbeat at this early gestational age is expected and normal, as cardiac activity typically becomes visible around 6 weeks gestational age or when the crown-rump length reaches approximately 5-7 mm 2, 3.
- The yolk sac, which is the first definitive marker of an intrauterine pregnancy, is usually visualized when the gestational sac exceeds 8 mm in mean sac diameter 1, 4.
- At 5 weeks, it is too early to diagnose pregnancy loss based on a single ultrasound, as the embryo and yolk sac may not yet be visible in a completely normal pregnancy 1.
Why Other Diagnoses Are Incorrect
A. Inevitable miscarriage is incorrect because:
- Inevitable miscarriage requires an open cervical os with bleeding, indicating that pregnancy loss is in progress 1.
- This patient has a closed cervical os with no bleeding, which excludes this diagnosis 1.
B. Threatened miscarriage is incorrect because:
- Threatened miscarriage requires vaginal bleeding with a closed cervical os in the presence of a viable intrauterine pregnancy 1.
- This patient has no bleeding, which is inconsistent with threatened miscarriage 1.
C. Pregnancy of unknown location is incorrect because:
- Pregnancy of unknown location (PUL) is defined as a positive pregnancy test with no evidence of intrauterine or ectopic pregnancy on transvaginal ultrasound 1.
- The 2025 Society of Radiologists in Ultrasound consensus clarified that any signs of an intrauterine pregnancy, whether probable or definite, would not qualify as a PUL 1.
- This patient has an intrauterine gestational sac visualized, which represents at least a probable intrauterine pregnancy and therefore excludes the diagnosis of PUL 1.
Critical Management Principles
Follow-up imaging is essential:
- Repeat transvaginal ultrasound should be performed in 7-14 days to assess for the appearance of a yolk sac, embryo, and cardiac activity 1, 5.
- Management decisions should never be based on a single ultrasound at this early gestational age, as this risks harm to a normal pregnancy 1.
Diagnostic criteria to avoid premature diagnosis of pregnancy loss:
- A gestational sac ≥25 mm in mean diameter without a visible embryo is required to diagnose anembryonic pregnancy 5, 6.
- An embryo ≥7 mm in crown-rump length without cardiac activity is required to diagnose embryonic demise 6, 2.
- At 5 weeks with a small gestational sac, neither of these criteria can be met, making the diagnosis of pregnancy loss inappropriate 1, 5.
Common Pitfalls to Avoid
- Do not diagnose pregnancy loss based solely on the absence of cardiac activity at 5 weeks, as this is expected in normal pregnancies at this early stage 1, 2.
- Do not use a single hCG level to guide management in the absence of definitive ultrasound findings, as serial measurements are required 1, 7.
- Guard against injury to normal pregnancies by avoiding overinterpretation of a single early ultrasound 1.