How accurate is a due date?
Quick answer
A due date is 280 days after the first day of the last period - a convention ACOG says assumes a 28-day cycle with day-14 ovulation, and does not account for recall error, irregular cycles or ovulation timing. ACOG and the NHS both describe term as 37 to 42 weeks.
Each of the three assumptions ACOG flags can be checked against data. On recall, ACOG reports that about half of women recall their last period accurately. On cycle length, Bull 2019 found a mean of 29.3 days and only 13 percent of cycles exactly 28 days across 612,613 cycles. On ovulation timing, the same study measured a mean luteal phase of 12.4 days with a 95 percent interval of 7 to 17 days.
Cycle length alone moves the estimate substantially. This site shifts it by cycle length minus 28, so for a last period on 1 January 2026 the estimate runs from 1 October for a 21-day cycle to 20 October for a 40-day cycle. That adjustment is arithmetic on ACOG's stated assumption rather than an ACOG recommendation, and it is labelled as a convention.
A first-trimester ultrasound supersedes all of it. ACOG treats a scan in that window as the most accurate method of establishing gestational age, and reports a study in which 40 percent of women were redated after a first-trimester scan differed from the last-period estimate by more than five days. The scan measures the pregnancy; the calculator infers it.
The most useful figure a due date calculator prints is not the date but the window around it. For an estimate of 8 October 2026, the 37-to-42-week term window runs 17 September to 22 October. How accurate are due date calculators works through each assumption in turn.
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