Your due date is 280 days (40 weeks) after the first day of your last period: Naegele's rule, the standard way to date a pregnancy (ACOG). Only about 1 in 20 women give birth on that day (ACOG), and most pregnancies last 37 to 42 weeks (NHS).
How your due date is calculated
Doctors and midwives date a pregnancy from the first day of your last menstrual period (LMP), not from the day you conceived. A typical pregnancy lasts about 280 days — 40 weeks — from that day (ACOG). This is Naegele’s rule, and it is the rule the Period Calc app uses too.
The rule assumes a 28-day cycle, with ovulation and conception about two weeks after your period started. If your cycles are usually longer or shorter, you probably ovulated later or earlier. The calculator keeps the standard due date whatever your cycle length, as the Period Calc app does, and uses your cycle length only to estimate when conception happened. That is why, with long or short cycles, the conception estimate and the due date are not always the usual 266 days apart.
The formula
Due date = first day of last period + 280 days (40 weeks)
Weeks pregnant = days since the first day of your last period ÷ 7
Conception ≈ first day of last period + cycle length − 14 days
A worked example
Your last period started on and your cycles are 28 days long.
- Due date
- ( + 280 days)
- Conception
- around
- On
- 12 weeks and 5 days pregnant, first trimester
- In the Hijri calendar
- last period on , due date
2026 has no ; in a leap year such as 2028, the same start date gives a due date of .
Weeks and trimesters
Pregnancy is counted in completed weeks and days: at 12 weeks and 5 days, you are in your 13th week. The first trimester lasts until 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks until birth (ACOG).
Due dates for four example start dates
Each due date is 280 days after the first day of the last period; the last two columns show how far along each pregnancy is, and its trimester, on .
Due dates for four example first days of the last period, with the Hijri date in brackets
| First day of last period | Due date | How far along | Trimester |
|---|---|---|---|
| () | () | 7 weeks, 3 days | First trimester |
| () | () | 16 weeks, 2 days | Second trimester |
| () | () | 29 weeks, 2 days | Third trimester |
| () | () | 37 weeks, 5 days | Third trimester |
How exact is a due date?
Only about 1 in 20 women give birth on their due date (ACOG). Most babies are born in the weeks around it: a pregnancy normally lasts from 37 to 42 weeks (NHS). An early ultrasound scan — the dating scan, at around 10 to 14 weeks (NHS) — measures the baby and can move your due date. If it does, follow the date from your scan.
When to see a doctor
Book an appointment with a midwife or doctor as soon as you know you are pregnant, and ask them if you are not sure when your last period started. If you have bleeding, pain or anything else that worries you, contact them straight away.
Not pregnant yet? The ovulation calculator shows your fertile days, and the period calculator your next periods. More about the app is in the Period Calc FAQ.
This calculator gives an estimate, not medical advice. Your doctor or midwife will confirm your due date.
Questions about your due date
Why is pregnancy counted from my last period and not from conception?
Most women know when their last period started but not the day they conceived. Counting from the period gives everyone the same starting point, which is why pregnancy is said to last 40 weeks.
What if I don’t remember the first day of my last period?
Enter your best guess and treat the result as approximate. An early ultrasound scan will give a more accurate date.
Does a long or short cycle change my due date?
This calculator, like the Period Calc app, uses the standard rule: 280 days from the first day of your last period, whatever your cycle length. Your cycle length only moves the conception estimate. If your cycles are much longer or shorter than 28 days, you may have ovulated later or earlier — your early scan, which dates the pregnancy by the baby’s size, gives the most accurate date.
Why is the Hijri date shown too?
So you can see your due date the way many families plan the year. The Hijri date uses the Umm al-Qura calendar and may differ by a day where you live.
Does the Period Calc app have a pregnancy mode?
Yes. Pregnancy mode shows your due date in Hijri and Gregorian, with your current week and trimester, and pauses period and ovulation reminders.
How this page was made
Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the health organisations listed under Sources. They have not reviewed it, and neither has a doctor. The calculator runs the formula above in your browser, with the same rules as the Period Calc app. It works from averages, so it cannot know how your own body differs from them, and it cannot diagnose anything: if something worries you, see when to see a doctor.