A 32-year-old woman on a levonorgestrel-releasing intrauterine system (LNG-IUS) for 3 years presents with amenorrhea and a positive urine pregnancy test. She reports no vaginal bleeding for the past 8 months. What is the most appropriate investigation to confirm the diagnosis and rule out ectopic pregnancy?

Explanation

## Investigation for Suspected Pregnancy with LNG-IUS in Situ ### Clinical Context This patient presents with: - Amenorrhea (expected with LNG-IUS but can mask pregnancy) - Positive urine pregnancy test - LNG-IUS in situ for 3 years - Risk of ectopic pregnancy (IUCDs increase relative risk of ectopic pregnancy if contraceptive failure occurs) **High-Yield:** LNG-IUS failure rate is ~0.2%, but when pregnancy occurs, the risk of ectopic pregnancy is significantly elevated (up to 50% of pregnancies with IUCD in situ are ectopic). ### Why Serum Beta-hCG and Transvaginal Ultrasound is the Answer **Key Point:** The combination of serum quantitative beta-hCG and transvaginal ultrasound is the gold standard for: 1. Confirming pregnancy (beta-hCG) 2. Determining location (intrauterine vs. ectopic) via TVS 3. Assessing viability and dating the pregnancy 4. Localizing the IUCD in relation to the gestational sac ### Diagnostic Algorithm for Pregnancy with IUCD in Situ ```mermaid flowchart TD A[Positive pregnancy test + IUCD in situ]:::outcome --> B[Serum beta-hCG + TVS]:::action B --> C{Intrauterine pregnancy?}:::decision C -->|Yes| D{IUCD location?}:::decision C -->|No| E[Ectopic pregnancy]:::urgent D -->|In situ| F[Counsel: remove IUCD + continue pregnancy]:::action D -->|Expelled| G[Continue pregnancy]:::action E --> H[Emergency management: MTX or surgery]:::urgent ``` ### Investigation Comparison Table | Investigation | Role | Sensitivity | Limitations | | --- | --- | --- | --- | | **Serum beta-hCG (quantitative)** | Confirms pregnancy; serial measurements assess viability | 100% at hCG >25 mIU/mL | Does not localize pregnancy | | **Transvaginal ultrasound** | Localizes pregnancy; detects ectopic; assesses viability; visualizes IUCD | >95% for intrauterine pregnancy at hCG >1000 mIU/mL | Operator-dependent; may miss early pregnancy if hCG <1000 | | **Transabdominal ultrasound** | Lower resolution; limited in early pregnancy | <90% for early pregnancy | Poor visualization of adnexa; not recommended as sole modality | | **Serum progesterone** | Assesses corpus luteum function; poor discriminator | Non-specific | Cannot differentiate intrauterine from ectopic pregnancy | | **Diagnostic curettage** | Historically used; now obsolete | — | Risks perforation, hemorrhage, and loss of viable intrauterine pregnancy; contraindicated | ### Key Findings on Transvaginal Ultrasound **Intrauterine Pregnancy:** - Gestational sac within endometrial cavity - Yolk sac visible at 5–6 weeks - Fetal pole with cardiac activity at 6–7 weeks - IUCD threads visible in cervical canal or lower uterine segment **Ectopic Pregnancy:** - No gestational sac in uterine cavity despite positive hCG - Adnexal mass (tubal pregnancy) or free fluid in pouch of Douglas - Requires emergency intervention **Clinical Pearl:** If intrauterine pregnancy is confirmed with IUCD in situ, the IUCD should be removed as soon as possible (ideally in first trimester) to reduce risk of spontaneous abortion (from ~50% to ~25%) and infection. The pregnancy can be continued safely after removal. **Mnemonic:** **BEAT** — Beta-hCG + Exam + Abdominal/Transvaginal ultrasound + Trending hCG (if needed) for pregnancy localization and viability assessment.