Pregnancy
How Is a Pregnancy Due Date Calculated? Naegele's Rule Explained
LMP + 280 days, the doctors' shortcut, adjusting for your cycle length and IVF, and a timeline of scans and trimesters.
By Team Vind · Updated · 5 min read
Your due date is the first date your doctor will give you, and every scan and test is timed from it. It's worked out from the first day of your last period, not the day you conceived. Here's how the calculation works, how to adjust it for your cycle, and why only about one baby in twenty arrives exactly on it.
Naegele's rule: LMP + 280 days
Due date = first day of last period + 280 days (40 weeks)
If your last period started on 10 March 2026, your due date is 15 December 2026. Pregnancy is counted from the period, about two weeks before conception, so at conception you are already "2 weeks pregnant".
The shortcut doctors use
Add one year, subtract three months, and add seven days. From 10 March 2026, that gives 17 December 2026. It can differ from the 280-day count by a day or two, depending on the months in between.
Try it: Pregnancy Calculator
Your Pregnancy
Key dates
| Milestone | Week | Date |
|---|---|---|
| Conception (about) | 2 | Sat, 15 August 2026 |
| First trimester ends | 13w 6d | Fri, 6 November 2026 |
| Anatomy scan (usually) | 18–20 | Sat, 5 December 2026 |
| Third trimester starts | 28 | Sat, 13 February 2027 |
| Full term | 37 | Sat, 17 April 2027 |
| Due date | 40 | Sat, 8 May 2027 |
| 42 weeks | 42 | Sat, 22 May 2027 |
If your cycle isn't 28 days
The 280-day rule assumes you ovulate on day 14 of a 28-day cycle. With longer cycles you ovulate later, so the due date moves later by the same number of days. With a 32-day cycle and the same period date, the due date is 19 December 2026, four days later.
Other ways to date a pregnancy
- Conception date: due date = conception + 266 days.
- IVF: from the transfer date, add 263 days for a 3-day embryo or 261 days for a 5-day blastocyst. IVF dates are the most accurate.
- Early ultrasound: a scan between 8 and 13 weeks measures the baby (crown-rump length). If it differs from the period date by more than about a week, doctors usually go by the scan.
Scan timeline for a 10 March 2026 period
| Milestone | Week | Around |
|---|---|---|
| First scan (dating, heartbeat) | 7 | 28 April 2026 |
| NT scan and first-trimester screening | 12 | 2 June 2026 |
| Second trimester begins | 14 | 16 June 2026 |
| Anomaly (level 2) scan | 19 | 21 July 2026 |
| Third trimester begins | 28 | 22 September 2026 |
| Full term | 37 | 24 November 2026 |
| Due date | 40 | 15 December 2026 |
Trying to conceive rather than already pregnant? Read how to find your fertile window. Gaining weight during pregnancy? The pregnancy weight gain calculator shows the healthy range for your BMI.
Frequently asked questions
Is the due date counted from conception or the last period?
From the first day of the last period. Conception is usually about two weeks later, so the due date is 266 days after conception.
Can the due date change?
Yes. If an early scan (8–13 weeks) differs from the period date by more than about a week, your doctor may change it to match the scan.
How many weeks is a full-term pregnancy?
37 to 42 weeks. Babies born from 37 weeks are considered full term; 40 weeks is the due date.
Which trimester am I in?
First trimester to the end of week 13, second from week 14 to 27, and third from week 28 until birth.
Sources
- American College of Obstetricians and Gynecologists: Methods for estimating the due date (Committee Opinion 700)
- Ministry of Health and Family Welfare: antenatal care guidelines
Last reviewed: 10 October 2026