Estimating Conception Date: Why LMP & Ultrasound Trump Calculators

There is no accurate "conception calculator" based solely on birth date. Conception date estimation requires:

Gold Standard: First-Trimester Ultrasound

• Crown-rump length (CRL) at 7–13 weeks: ±3–5 day accuracy.
• Dating scan establishes gestational age (GA) → conception = GA − 2 weeks (assuming 14-day follicular phase).

LMP-Based Estimation (If No Early Ultrasound)

• Conception ≈ LMP + 14 days (for 28-day cycles).
• Adjust for cycle length: Ovulation ≈ LMP + (cycle length − 14). Conception window: ovulation day ±1 day.
• Example: LMP Jan 1, 32-day cycle → ovulation ~Jan 19 → conception Jan 18–20.

Why Birth Date Alone Fails

• Gestation varies: 37–42 weeks (term range).
• Ovulation timing varies cycle-to-cycle.
• Sperm survival: up to 5 days in female tract.

Clinical Takeaway

If you need a precise conception date (paternity, dating discrepancy, recurrent loss workup), obtain a first-trimester dating ultrasound. Retrograde calculation from delivery is speculative (±2–3 weeks).