Pregnancy Due Date Calculator
Enter the first day of your last menstrual period to estimate your due date, how many weeks along you are, and which trimester you're in.
A due date gives you and your care team a target to plan around, even though only about one in twenty babies actually arrives on the exact day. The most common way to estimate it is Naegele's rule, which counts 280 days — 40 weeks — from the first day of your last menstrual period. This calculator applies that rule, adjusts for your cycle length, and also shows how many weeks along you are today and which trimester you're in.
How the due date is estimated
Pregnancy is dated from the first day of your last period, not from conception, which usually happens about two weeks later. Adding 280 days gives the estimated due date; if your cycle is longer or shorter than 28 days, the date shifts accordingly.
(adjusted by cycle length − 28 days)
Weeks along = Today − first day of last period
| Milestone | Weeks |
|---|---|
| First trimester | 0 – 13 weeks |
| Second trimester | 14 – 27 weeks |
| Third trimester | 28 – 40 weeks |
| Full term | 37 – 42 weeks |
Worked example
If your last period began on 1 April with a 28-day cycle:
| Due date = 1 April + 280 days | ≈ 6 January |
| Estimated conception | ~15 April |
Why the date may change
Dating from the last period assumes a regular 28-day cycle and ovulation on day 14, which isn't true for everyone. That's why an early "dating" ultrasound, usually between 8 and 14 weeks, is considered more accurate and may move your due date by a few days. Only around 4% of babies are born on their due date, and a birth anytime from 37 to 42 weeks is considered full term. Your midwife or doctor is the best source for a confirmed date and personalised care.