How a Pregnancy Due Date Is Estimated: The 280-Day Rule
The first question after a positive test is usually "when?". The due date also sets the schedule for everything that follows: when to book the first appointment, when the anatomy scan happens, when to tell work. Due date estimates it from whatever date you know and lays out the main milestones. It's a date calculation, not medical advice, and your doctor or midwife may set a different date.
Steps
- Choose what you know: last period, conception date, or IVF transfer.
- Enter the date, plus your usual cycle length if you're starting from the last period.
- Read the due date, how far along you are today, and the milestone dates.
How the date is calculated
- From the last menstrual period (LMP): add 280 days (Naegele's rule), then adjust for cycle length. A 28-day cycle is assumed by default; a 32-day cycle adds 4 days. Cycle lengths from 20 to 45 days are accepted.
- From a conception date: add 266 days.
- From an IVF transfer: add 263 days for a day-3 embryo, or 261 days for a day-5 (blastocyst) embryo.
Pregnancy is counted from the first day of the last period, not from conception, which is why "40 weeks" includes roughly two weeks before conception happened.
What else it shows
- Weeks and days pregnant today, and the trimester.
- End of the first trimester.
- The 18 to 22 week anatomy scan window.
- Start of the third trimester.
- And the estimated due date itself.
A due date is an estimate
Only a small share of babies arrive on their due date. Births cluster in the weeks around it, and a baby born between 37 and 42 weeks is generally considered on time. Treat the date as the centre of a window.
Your provider will often confirm or adjust it with an early ultrasound, which measures the baby and can be more accurate than LMP, especially with irregular cycles. If the scan gives a different date, use theirs.
Example
Last period started 1 February 2026, regular 30-day cycle. 1 February + 280 days = 8 November 2026, plus 2 days for the longer cycle = 10 November 2026. By late October that pregnancy is around 38 weeks along, well into the third trimester, and the page's Pregnancy today line shows the exact weeks and days.
Practical uses
- Planning appointments around the scan windows.
- Telling an employer with a reasonable date for maternity leave (in India, the Maternity Benefit Act and your employer's policy set the rules; check both).
- Family planning around travel, festivals and work commitments.
Rules as of October 2026
Rules as of 27 October 2026. The page adds 280 days to the first day of the last menstrual period, and says plainly that this is a date calculation, not medical advice. Your healthcare provider may set or change your due date, often based on an ultrasound. Follow their dates and advice.
Official sources:
Privacy
According to the page, the date isn't uploaded; like other site settings it may be remembered on this device so a reload keeps it, and Reset clears it. The page states clearly that it isn't medical advice. Google ads appear on the page, as on the rest of the site.
FAQ
Is the due date the day I'll give birth? No, it's the centre of a window; births cluster around it.
What if I know the conception date? Choose that option; the page adds 266 days.
Does it adjust for a long cycle? Yes, from 20 to 45 days.
How many weeks pregnant am I? The Pregnancy today line shows weeks, days and trimester.
Get an estimate, then confirm it with your provider: Due date.
— Alok Kumar Singh