How the due date is calculated
- From your last period: add 280 days (40 weeks) to the first day of your last period. If your cycle is longer or shorter than 28 days, we adjust by the difference (Naegele's rule).
- From conception: add 266 days (38 weeks).
- IVF: add 261 days to a day-5 transfer or 263 days to a day-3 transfer.
Pregnancy trimesters
| Trimester | Weeks |
|---|---|
| First trimester | Week 1 to week 13 |
| Second trimester | Week 14 to week 27 |
| Third trimester | Week 28 to birth |
After your first scan
An early ultrasound, especially between 8 and 13 weeks, is the most accurate way to date a pregnancy. If it differs from your period-based date, your doctor will use the scan date.
Frequently asked questions
How accurate is a due date calculator?
It gives a good estimate, but only about 4–5% of babies are born on their due date. Most arrive between 38 and 42 weeks.
How are pregnancy weeks counted?
From the first day of your last period, about two weeks before conception. That is why you are already ‘4 weeks pregnant’ when a test turns positive.
What if I don't know my last period date?
Your doctor can date the pregnancy with an ultrasound.
Sources
- American College of Obstetricians and Gynecologists. Methods for estimating the due date (Committee Opinion 700), 2017. Link
Medical disclaimer: This calculator gives estimates for general information only. It is not a diagnosis or a substitute for advice from a doctor or registered dietitian. Please consult a healthcare professional before making changes to your diet, exercise or medicines, especially if you are pregnant, have a medical condition or take regular medication.
Last updated: 20 October 2026. Read our editorial policy.
