Pregnancy Due Date Calculator
Estimated due date from your last menstrual period.
Estimate a pregnancy due date using Naegele's rule — 280 days from the first day of your last menstrual period — the standard method used in obstetrics.
How your due date is calculated
Enter the first day of your last menstrual period (LMP). The calculator adds 280 days (40 weeks) to that date, and also shows how many weeks and days pregnant you are today.
Example: if your last period started on March 1, 2026, your estimated due date is Sunday, December 6, 2026.
Pregnancy is counted from the start of your last period — about two weeks before conception — so at conception you're already considered about 2 weeks pregnant.
Pregnancy by trimester
| Trimester | Weeks | What often happens |
|---|---|---|
| First | Weeks 1–13 | First prenatal visit, dating ultrasound, early screening tests |
| Second | Weeks 14–27 | Anatomy scan around weeks 18–22; many people first feel the baby move |
| Third | Week 28 to birth | More frequent checkups; glucose screening usually happens around weeks 24–28 |
When babies actually arrive
- Early term: 37 weeks to 38 weeks 6 days
- Full term: 39 weeks to 40 weeks 6 days
- Late term: 41 weeks to 41 weeks 6 days
- Post-term: 42 weeks and beyond
Most babies are born within about two weeks of their due date, so it's best to think of it as a due window rather than a single day.
If your cycle isn't 28 days
The 280-day rule assumes a regular 28-day cycle with ovulation around day 14. If your cycles are longer, shorter or irregular, or if you don't remember the date of your last period, your provider will usually set the due date from an early ultrasound instead, which is more accurate in the first trimester. IVF pregnancies are dated from the embryo transfer date.
Frequently asked questions
How is the due date calculated?
Using Naegele's rule: 280 days (40 weeks) after the first day of your last menstrual period (LMP) — the standard estimation method used in obstetrics.
How accurate is this?
It's a widely used estimate, but only about 5% of babies are actually born on their exact due date — most arrive within a couple of weeks either side. An ultrasound dating scan is more precise than this calculation alone.
Is this a substitute for medical advice?
No — always confirm dates and progress with a doctor or midwife. This tool is for general reference only.
Can my due date change?
Yes. It's common for a doctor or midwife to adjust the date after an early ultrasound, especially if the measurements differ from the LMP estimate by more than about a week.
How many weeks pregnant am I?
Enter the first day of your last period above — the calculator shows how many weeks and days along you are today, counted from that date.
This tool provides a general estimate and isn't medical advice.