Forty weeks is not ten calendar months: avoid pregnancy-date shortcuts
Weeks contain seven days, while calendar months vary in length. Day-based arithmetic avoids the errors caused by treating every month as four weeks.
Due date calculator →Compare the two operations
Forty times seven is 280 days. Starting at 2026-01-01, adding 280 days gives 2026-10-08. Adding ten calendar months instead gives 2026-11-01. These are different operations, so they should not be expected to agree. Replacing each month with four weeks also accumulates a difference over a long interval.
Check the week count using a date interval
From 2026-01-01 to 2026-01-29, the interval is 28 days, or four weeks. Be explicit about whether an endpoint is being counted as an additional calendar date. The date-difference tool reports the interval with the end date excluded; adding an extra day by habit changes the question being answered.
Keep calendar precision separate from clinical certainty
An estimated due date is not a promise that birth will occur on that day. Calendar tools help explain records and check arithmetic, but personal gestational dating and follow-up plans should use the information established by the maternity-care team. Do not change appointments or delay care because an independent month-based shortcut gives a different result. The date-addition tool is a useful arithmetic cross-check only.
Try the calculators
- Due date calculator
Estimate a due date from the first day of the last period.
- Date difference
Count calendar days between two dates.
- Add days to date
Find the date before or after a number of days.