Due Date by Last Period vs. by Conception Date: Why They Can Differ
The last-menstrual-period method adds 280 days from the period start date, assuming ovulation around day 14 of a standard cycle. A conception-date method adds roughly 266 days from a known or estimated conception date instead — the two generally align for a typical 28-day cycle, but can diverge for anyone with a different cycle length or a precisely known conception date, such as through fertility treatment.
This is general educational information about the two common estimation approaches — a healthcare provider can advise which method is most appropriate for a specific situation.
The last-menstrual-period method
This is the most commonly used default method, since the start date of the last period is generally easier to identify than an exact conception date — it works reasonably well for someone with a typical, regular cycle close to 28 days, but becomes less precise for a notably different cycle length.
The conception-date method
When the conception date is known with more precision — for example, through fertility treatment with a documented procedure date — calculating from that date directly (roughly 266 days to the estimated due date) can produce a more accurate estimate than the standard cycle-based assumption, since it doesn't depend on assuming a typical cycle.
Why the two methods can diverge
For someone with a longer or shorter cycle than the standard 28-day assumption, actual ovulation and conception likely occurred earlier or later than the last-period method assumes — in these cases, a conception-date-based or ultrasound-based estimate can differ meaningfully from the standard last-period calculation.