Conception Date Calculator: Estimating from Due Date

Conception date is estimated by subtracting 266 days (38 weeks) from the due date, not 280 days.

Calculation

• Due Date: April 18, 2023
• Gestational Age at Term: 280 days (40 weeks) from LMP
• Conception Occurs ~Day 14 of Cycle: So conception date = Due Date – 266 days

Correct Calculation

April 18, 2023 – 266 days = July 26, 2022

Why 266, Not 280?

• 280 days = LMP to due date (Naegel's rule)
• Conception ~14 days after LMP → 280 – 14 = 266 days from conception to due date

Clinical Nuances

• Ovulation timing varies (cycle length, irregularity)
• Ultrasound dating (CRL 7–13 weeks) is gold standard (±3–5 days)
• IVF: Known conception date (retrieval/transfer)

For a due date of April 18, 2023, conception likely occurred around July 26, 2022 (±1 week).