The estimate is based on ovulation timing, not the period itself
For a standard 28-day menstrual cycle, ovulation typically occurs around day 14, counting from the first day of the last menstrual period (LMP). Conception can only happen around ovulation, since that's when an egg is available for fertilization — so the estimated conception date is simply LMP + 14 days, with a window of a few days on either side to account for sperm survival time and natural variation in exactly when ovulation occurs within that cycle.
A worked example
For an LMP of 1 January 2026, the estimated conception date is 15 January 2026 (14 days later), with an estimated fertile window running from 12 January to 18 January 2026 — a 7-day window centered on the estimated ovulation date, reflecting that sperm can survive several days awaiting ovulation and that ovulation timing itself isn't perfectly fixed.
How this relates to the due-date calculation
Both the due-date estimate (LMP + 280 days) and the conception-date estimate (LMP + 14 days) start from the identical reference point — the LMP — but answer different questions. The due date estimates when the pregnancy will end; the conception date estimates when it began. Because both are anchored to the same LMP date, they move together: a due date calculated from a given LMP and a conception date calculated from that same LMP are two different views of one consistent timeline, not independent estimates that could contradict each other.
Why this is a window, not a single precise date
Even with a perfectly regular 28-day cycle, ovulation timing has natural day-to-day variability, and sperm can remain viable for several days before ovulation occurs — which is why conception is estimated as a window rather than a single exact day, unlike the due-date calculation, which is conventionally expressed as one specific date despite carrying similar underlying uncertainty.
What can make this estimate less accurate
This method assumes a standard 28-day cycle with ovulation on day 14 — cycles shorter or longer than 28 days shift the actual ovulation date earlier or later than this estimate assumes, since ovulation timing is what actually varies between people, not the length of the second half of the cycle. Anyone with a known irregular cycle, or tracking ovulation directly (e.g. via basal body temperature or ovulation predictor kits), will generally get a more accurate conception-window estimate from that direct tracking than from this LMP-based calculation alone.