How the conception calculator works
The biology of the window
Conception requires sperm meeting egg in a narrow window: the egg survives ~12–24 hours after ovulation, sperm survive up to 5 days beforehand. The result: sex from ~5 days before ovulation through the day of produces nearly all conceptions — and the "conception date" is really the ovulation date ±. The calculator reports the window for exactly this reason.
Reverse-dating from a due date
Working backward: due date − 266 days ≈ conception; − 280 ≈ LMP; +14 from LMP ≈ ovulation. Each step carries its own uncertainty (cycle length, sperm survival), which is why the output is a range with a center. For pregnancy-dating purposes this is plenty precise; for other questions, precision requires different tools.
Cycle-length adjustments
The 14-day post-LMP ovulation assumes a 28-day cycle. Longer cycles ovulate later (a 35-day cycle ovulates ~day 21); shorter, earlier. The fertile window shifts with the cycle, and irregular cycles move it unpredictably — the main reason ultrasound dating supersedes all of this arithmetic once a scan exists.