Conception Date Calculator — Reverse-Compute From EDD or Birth Date With 6-Day Fertile Window

A Brief History of Conception Dating

In 2026, a couple in Bangalore want to know when their first child was conceived. The baby was born on May 15 2026. Subtracting 266 days (the standard 40-week term minus 14 days from LMP to conception) gives August 22 2025 — and the 6-day fertile window from August 17 to August 22 2025. The calendar SVG above shows the egg release on cycle day 14 of the August 2025 cycle. That entire calculation rests on three centuries of reproductive biology.

Reinier de Graaf, the Delft anatomist, first described the Graafian follicle in 1672 — the fluid-filled ovarian structure that releases the ovum. Karl Ernst von Baer photographed the mammalian ovum in 1827, settling a century-long debate (preformationism vs epigenesis) and establishing the modern view of conception. The half-cycle interval between LMP and ovulation (~14 days) was demonstrated by Knaus and Ogino independently in 1928-1932, the same work that birthed the rhythm method.

The 28-day average cycle was first systematically measured by Treloar et al in 1967 (n=2702 women, 25,825 cycles): mean 28.1 days with 95% CI 23-35 days. The 14-day luteal phase is remarkably constant within individuals; the follicular phase varies and explains most of the cycle-length variation. So in irregular cycles, ovulation is approximately 14 days BEFORE the next period, not 14 days after the last - a key insight for reverse-computing conception.

Allen Wilcox and colleagues at the US NIH National Institute of Environmental Health Sciences published the landmark 1995 NEJM paper Timing of sexual intercourse in relation to ovulation (n=625 healthy couples planning pregnancy, daily urinary metabolites + sexual diary). They established the 6-day fertile window: pregnancy can result from intercourse occurring 5 days before ovulation up to the day of ovulation itself, with per-day probability rising from ~10% at -5 days to a peak of ~33% on ovulation day.

IVF inverted the conception-dating problem. From Louise Brown's birth in 1978 (Steptoe and Edwards, Oldham), the conception moment was directly observed for the first time: egg retrieval, in-vitro fertilization, and embryo transfer were calendarised. The Society for Assisted Reproductive Technology (SART) 2024 dataset shows the modern standard: day-5 blastocyst transfer with EDD = transfer + 261 days, accurate to the day.

Modern obstetric coding tools (ICD-11 chapter 17 codes, WHO 2022) carry conception date as a separate field from LMP and EDD. ACOG Committee Opinion 700 (2017, reaffirmed 2024) gives the formal dating priority: known IVF date > first-trimester ultrasound CRL > LMP-derived calculation. This page's 'From EDD' mode handles all three by inverting whichever EDD method was used.

For forward computation use Due Date Calculator. To time the fertile window for an upcoming pregnancy use Fertility Window.