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Due date = LMP + 280 days, so conception ≈ LMP + 14 days. Working backward from due date: conception = due date − 266 days. Fertilization window is 3–5 days before ovulation through 1 day after. Ultrasound crown-rump length in the first trimester is more accurate than LMP for dating, especially with irregular cycles. Paternity testing is the only definitive way to determine biological paternity.
Approximate conception date from due date (266 days).
January 15, 2026