Pregnancy Due Date Calculator
Estimate a due date from the last period or the conception date.
Good to know. Only about 1 in 25 babies arrives on the due date. Think of it as a due month.
Not medical advice. Only your midwife or doctor can date a pregnancy.
How it works
In plain words
Pregnancy is dated from the first day of the last period, and lasts about 280 days (40 weeks). If your cycle is longer or shorter than 28 days, ovulation shifts by the difference. From conception, it is about 266 days.
The formula
due date = last period + 280 days + (cycle length − 28) due date = conception + 266 days
Worked example
If the last period started on 1 January 2026 with a 28-day cycle, the estimated due date is 280 days later: Thursday, October 8, 2026.
Sources
- ACOG Committee Opinion 700, Methods for Estimating the Due Date, 2017 280 days from the first day of the last period (Naegele’s rule). An early ultrasound is more accurate.
Sources checked September 29, 2026. 5 automated tests check its answers on every change.
About the pregnancy due date calculator
Due dates are usually worked out with Naegele’s rule: take the first day of the last menstrual period and add 280 days, or 40 weeks. This assumes a regular 28-day cycle with ovulation around day 14. If your cycle is usually longer or shorter, ovulation tends to happen later or earlier, so the calculator shifts the date by the difference.
This is why pregnancy is counted from before conception. At the moment of conception, you are already considered about 2 weeks pregnant. If you know the conception date, for example after IVF, the due date is about 266 days (38 weeks) after it.
The due date is an estimate, not a deadline. Only about 1 in 20 babies are born on their due date. Most arrive in the weeks either side, and anything from 37 to 42 weeks is considered normal.
In most countries, an early ultrasound scan, usually between 8 and 14 weeks, gives a more accurate date than any calculation from a period, especially if cycles are irregular. Once you have a scan date, use that one.
The "how far along" figure uses today’s date. If you have any questions or concerns about your pregnancy, contact your midwife, doctor or local maternity service.
Questions people ask
How is a due date calculated?
By adding 280 days (40 weeks) to the first day of the last period, adjusted if your cycle isn’t 28 days. From a known conception date, add 266 days.
How accurate is a due date?
It is an estimate. Only about 5% of babies arrive on the day, and most are born within two weeks either side. An early ultrasound is more accurate.
Why does pregnancy count from my last period?
Because the date of the period is usually known, and conception typically happens about two weeks later. It is the standard medical convention.
What if my cycles are irregular?
Date calculations are less reliable. Your midwife or doctor will usually use an ultrasound scan to date the pregnancy.