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Estimated Conception Date from LMP

Estimated conception (ovulation) date = first day of the last menstrual period + 14 days, adjusted by (cycle length − 28) days. It is an estimate: fertilisation can occur up to about 5 days after intercourse.

  • First day of last menstrual period (LMP)
  • Usual cycle length
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Formula

Conception ≈ LMP + 14 days + (cycle length − 28) days

Clinical pearls

  • First-trimester ultrasound dating is more accurate than LMP for estimating conception.
  • MedCalc-Bench uses LMP + 14 days without cycle adjustment; this tool gives the same answer for a 28-day cycle (the default) and shifts it for other cycle lengths.

Examples

conception date lmp 2024-01-01when did I conceive if my last period started March 3 2025 with a 32 day cycle

FAQ

How do you work out the conception date?

Add 14 days to the first day of the last period for a 28-day cycle; for longer or shorter cycles add or subtract the difference from 28.

Can a conception date prove paternity?

No. The estimate has an error of several days, and sperm can survive up to 5 days, so dates alone cannot establish paternity.

Evidence

  1. ACOG Committee Opinion No 700: Methods for Estimating the Due Date. Obstet Gynecol 2017;129:e150–4.
  2. MedCalc-Bench reference implementation (estimated_conception_date.py).

Pediatrics & Obstetrics · Reviewed 2026-10-14 · Methodology