## Investigation of Choice for Emergency Contraception Eligibility ### Clinical Context Emergency contraception (EC) must be offered within a specific window post-intercourse. Before initiating any hormonal EC, it is essential to exclude pre-existing pregnancy, as EC is contraindicated in pregnancy. ### Why Serum β-hCG is the Investigation of Choice **Key Point:** Serum β-hCG is the gold standard to rule out pregnancy before prescribing levonorgestrel or any hormonal emergency contraceptive. **High-Yield:** - β-hCG becomes detectable 6–8 days after ovulation (approximately 3–4 days after conception) - At 36 hours post-intercourse, if conception has occurred, hCG levels may still be below the detection threshold of urine pregnancy tests - Serum β-hCG is more sensitive (detects hCG as low as 1–5 mIU/mL) than urine tests (typically 20–25 mIU/mL) - A negative serum β-hCG confirms the woman is not pregnant and EC is safe **Clinical Pearl:** Even though the patient is on day 12 of her cycle (likely pre-ovulation), pregnancy from a previous cycle must be ruled out. Levonorgestrel is a progestin and may cause teratogenic effects if pregnancy is already established. ### Mechanism of EC Action Levonorgestrel works by: 1. Delaying or inhibiting ovulation (if given before the LH surge) 2. Altering cervical mucus viscosity 3. Impairing endometrial receptivity It does NOT interrupt an established pregnancy (no abortifacient effect). ### Timing and Efficacy - **Most effective:** Within 72 hours of unprotected intercourse - **Can be used:** Up to 120 hours (5 days), though efficacy decreases with time - At 36 hours, the patient is well within the window for maximum efficacy ### Contraindications to EC - Established pregnancy (confirmed by positive hCG) - Absolute contraindications to hormonal methods (e.g., migraine with aura, uncontrolled hypertension, VTE history) [cite:Park 26e Ch 9]
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