How accurate are due date calculators?

What the calculator is actually doing

It adds 280 days to the first day of your last menstrual period. That is the whole method, and it is the convention ACOG sets out in Committee Opinion No. 700. The interesting part is what ACOG says next: the convention assumes a regular 28-day cycle with ovulation on the 14th day, and it does not account for inaccurate recall of the last period, irregular cycle length, or variation in when ovulation actually happened. Three assumptions, stated by the body that publishes the rule, in the document that publishes it.

How often each assumption holds

Not as often as the tidiness of the arithmetic suggests. ACOG reports that about half of women recall their last period accurately. On cycle length, the largest real-world dataset - 612,613 cycles from 124,648 users, published by Bull and colleagues in 2019 - found a mean cycle of 29.3 days and only 13 percent of cycles exactly 28 days long. 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. The day-14 assumption is a midpoint of a ten-day span, not a constant.

Why a first-trimester scan supersedes the arithmetic

ACOG treats a first-trimester ultrasound as the most accurate method of establishing gestational age, and reports that in one study 40 percent of women had their due date adjusted after a first-trimester scan because it differed from the last-period date by more than five days. That is not the scan being wrong or the calculator being broken. A scan measures the pregnancy; the calculator infers it from a date you remembered and an assumption about your cycle. When the two disagree, the measurement is the one that stands.

The cycle-length adjustment, and what it is not

A longer cycle usually means later ovulation, so the same last period implies a later due date. This site shifts the estimate by your cycle length minus 28, which is arithmetic performed on ACOG's own stated assumption rather than an ACOG recommendation - the formula pack labels it a convention for exactly that reason. It also assumes the whole difference in cycle length falls in the follicular phase, which is where Bull 2019 found most of the variation, though the luteal phase varies too.

A window, not an appointment

The most useful thing a due date calculator prints is not the date. ACOG states plainly that only about 1 in 20 women give birth on their due dates, and both ACOG and the NHS describe 37 to 42 weeks as term - which for a due date of 8 October 2026 means 17 September to 22 October, a five-week window. That is why this site prints the term dates and the trimester boundaries alongside the estimate, and why it treats a due date as the midpoint of a distribution rather than a scheduled event.

Official sources

• ACOG Committee Opinion No. 700, Methods for Estimating the Due Date, Obstet Gynecol 2017
• Bull et al., Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles, npj Digital Medicine 2019
• ACOG, Fertility Awareness-Based Methods of Family Planning (patient FAQ)
• ACOG, How Your Fetus Grows During Pregnancy (patient FAQ)

Frequently asked questions