A pregnancy lasts about 280 days (40 weeks), counted from the first day of your last menstrual period (LMP), not from the day of conception. This is how doctors work out your estimated due date (EDD), a method known as Naegele's rule.
The due date is an estimate. Only a small share of babies arrive exactly on it; most are born between 37 and 42 weeks. Your doctor may adjust the date after an early ultrasound scan.
How to use the due date calculator
- Choose how you want to calculate: from your last period, your conception date or an IVF embryo transfer.
- Enter the date.
- If you use your last period, enter your usual cycle length (28 days if you are not sure).
You will see your due date, how far along you are today, your trimester, the days left and the likely conception date. Open the table below the result for key dates such as the start of each trimester.
How the due date is calculated
From last period: Due date = LMP + 280 days + (cycle length − 28 days)
From conception: Due date = conception date + 266 days
IVF, 3-day embryo: Due date = transfer date + 263 days
IVF, 5-day embryo: Due date = transfer date + 261 daysA longer cycle usually means later ovulation, so the due date moves later by the same number of days.
Example
Your last period started on 1 March 2026 and your cycle is 28 days.
- Due date: 1 March 2026 + 280 days = Sunday, 6 December 2026
- Likely conception: around 15 March 2026
- On 24 September 2026 you would be 29 weeks 4 days pregnant, in the 3rd trimester
With a 32-day cycle, the due date moves 4 days later, to 10 December 2026.
Trimesters and term
| Stage | Weeks |
|---|---|
| 1st trimester | Week 1 to 13 (up to 13 weeks 6 days) |
| 2nd trimester | Week 14 to 27 |
| 3rd trimester | Week 28 until birth |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post-term | 42 weeks and beyond |
These are the definitions used by obstetricians. A baby born before 37 weeks is called preterm.
Important
- Visit a doctor as soon as you think you are pregnant, and follow their schedule for check-ups and scans.
- If your periods are irregular or you don't remember the date, an early ultrasound gives a more reliable due date.
- This calculator is for information only and does not replace medical care.
FAQ
How accurate is the due date?
It is an estimate. Most babies are born within two weeks either side of the due date. An ultrasound in the first trimester is the most accurate way to date a pregnancy, so your doctor's date may differ slightly from this one.
Why are weeks counted from my last period and not from conception?
The date of conception is usually unknown, but most women know when their last period started. So pregnancy is counted from that day, which means you are already about 2 weeks "pregnant" at the time of conception.
My scan gives a different due date. Which one is right?
Follow your doctor. If an early scan differs noticeably from the date based on your period, doctors usually go by the scan.
Can I use this if my periods are irregular?
It will be less accurate. If you know your conception date or IVF transfer date, choose that option instead. Otherwise your doctor will date the pregnancy with an ultrasound.
How many weeks is a full-term pregnancy?
A pregnancy is full term from 39 weeks 0 days to 40 weeks 6 days. Babies born from 37 weeks are early term, and before 37 weeks are preterm.