How is a due date calculated?
Doctors usually count pregnancy from the first day of your last menstrual period (LMP), not from conception. Your estimated due date is 280 days (40 weeks) after your LMP for a 28-day cycle — this is known as Naegele’s rule. If your cycle is longer or shorter than 28 days, we move the date by the difference, because you probably ovulated later or earlier.
- From your last period: LMP + 280 days (± your cycle length − 28)
- From conception: conception date + 266 days (38 weeks)
- From IVF: day-5 embryo transfer + 261 days, or day-3 transfer + 263 days
An early ultrasound (before about 14 weeks) is the most accurate way to date a pregnancy, so your doctor may change your due date after your dating scan.
Pregnancy milestones
| Week |
What happens |
| 6–9 |
First prenatal visit; heartbeat often visible on ultrasound |
| 11–14 |
Dating / nuchal translucency (NT) scan and first-trimester screening |
| 13 |
Second trimester begins |
| 18–22 |
Anatomy (anomaly) scan |
| 24–28 |
Glucose test for gestational diabetes |
| 28 |
Third trimester begins |
| 37 |
Full term begins (37–40 weeks) |
| 40 |
Due date |
Only about 4–5% of babies arrive on their exact due date. Most are born between 37 and 42 weeks.
Now that you know your due date, see our trimester-wise pregnancy diet plan.
Frequently asked questions
How accurate is a due date calculator?
It gives an estimate. Only about 1 in 20 babies is born on the due date itself. An early ultrasound is more accurate, especially if your cycles are irregular or you are unsure of your last period.
Why is pregnancy counted from my last period?
Because the date of ovulation and conception is usually not known, while the first day of the last period is. So at conception you are already counted as about 2 weeks pregnant.
Can my due date change?
Yes. If your first ultrasound measures your baby as noticeably older or younger than your period dates suggest, your doctor may update your due date.
How is an IVF due date calculated?
Add 261 days to a day-5 (blastocyst) transfer date, or 263 days to a day-3 transfer date. Your fertility clinic will confirm the date.
Sources: ACOG: Methods for estimating the due date · NHS: Your pregnancy and baby guide
Reviewed for accuracy against the sources above. This page is general information, not medical advice — talk to your doctor or a registered dietitian about your own needs.
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