Pregnancy Due Date: The 280-Day Estimate, Worked by Hand

"Your due date is January 6" sounds precise, but it is built from one simple rule applied to one date: the first day of the last period. Add 280 days — exactly 40 weeks — and that is the estimate. No conception date, no scan, no ultrasound measurement. Just one addition, worked by hand below.

The method: add 280 days to the last period

due date = first day of last period + 280 days

280 days is exactly 40 weeks (280 ÷ 7 = 40, no remainder), which is why pregnancy is commonly described as "40 weeks" even though it is dated from the last period, not from conception — conception itself typically happens about two weeks after the last period starts, so the 40-week count already has a two-week head start built in. This is Naegele's rule, the standard estimate behind most due-date calculations.

A worked example: last period April 1, 2026

April has 30 days, so from April 1 to April 30 is 29 days. Add each full month after that: May (31), June (30), July (31), August (31), September (30), October (31), November (30), December (31). Running total: 29 + 31 = 60 (end of May), + 30 = 90 (end of June), + 31 = 121 (end of July), + 31 = 152 (end of August), + 30 = 182 (end of September), + 31 = 213 (end of October), + 30 = 243 (end of November), + 31 = 274 (end of December). That leaves 280 − 274 = 6 days into January: the estimated due date is January 6, 2027, a Wednesday.

A second example: last period January 15, 2026

January has 31 days, so 31 − 15 = 16 days remain in January. Add the following months: February (28, since 2026 is not a leap year), March (31), April (30), May (31), June (30), July (31), August (31), September (30). Running total: 16 + 28 = 44, + 31 = 75, + 30 = 105, + 31 = 136, + 30 = 166, + 31 = 197, + 31 = 228, + 30 = 258 (end of September). Remaining: 280 − 258 = 22 days into October: October 22, 2026, a Thursday.

A third example: crossing a year boundary

Last period December 25, 2025. December has 31 days, so 31 − 25 = 6 days remain in December. Add the following months into the new year: January (31), February (28), March (31), April (30), May (31), June (30), July (31), August (31), September (30). Running total: 6 + 31 = 37, + 28 = 65, + 31 = 96, + 30 = 126, + 31 = 157, + 30 = 187, + 31 = 218, + 31 = 249, + 30 = 279 (end of September 2026). Remaining: 280 − 279 = 1 day into October: October 1, 2026, a Thursday — the year boundary in between needed no special treatment, since it is just one more day in a running count of months.

Why 280 days, and why it is only an estimate

280 days assumes a 28-day cycle with ovulation around day 14, which is an average, not everyone's actual cycle. The tool's own numbers put this in perspective: only about 1 in 20 births land exactly on the predicted day. The estimate is still useful — it sets a rough window for the third trimester, scheduling and leave — but a clinician can revise it with an ultrasound measurement, which is more accurate than a last-period date alone, especially for anyone with an irregular cycle.

Not medical advice

This is informational only. It estimates a date from one number — the first day of the last period — and nothing about the pregnancy itself. Any decision about care, timing or symptoms belongs with a clinician, not a date this addition produces.

What the tool adds

The addition above — 280 days from the last period, walked month by month — is all you need, on paper, to estimate a due date. The tool's job is just speed and not tracking a running total across nine months by hand: enter the date, and it prints the estimate instantly, entirely on your device.