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The moment a pregnancy is confirmed, one question comes up again and again: “When is the baby due?” The answer is calculated in a surprisingly simple way — yet the date can still shift after your first scan. Here’s how it works.
This article is general information, not medical advice. Your doctor or midwife will confirm your dates and guide your care.
Method 1: from your last period (Naegele’s rule)
Most due dates start with the first day of your last menstrual period (LMP). Add 280 days — exactly 40 weeks — and you have the estimated due date (EDD).
A popular shortcut, known as Naegele’s rule: take the first day of your last period, add one year, subtract three months, and add seven days.
Example: last period started on 1 May 2026.
Add one year → 1 May 2027. Subtract three months → 1 February 2027. Add seven days → 8 February 2027.
Counting exactly 280 days gives 5 February 2027 — the shortcut can be a few days off because months have different lengths.
What if your cycle isn’t 28 days?
The rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are regularly longer, you probably ovulated later, so the due date moves later by the same number of days. A 32-day cycle typically shifts the date four days later.
Method 2: from a known conception date
If you know when you conceived — for example from ovulation tracking — the due date is 266 days (38 weeks) after conception. It lands on the same day as the LMP method, because conception usually happens about two weeks after the period starts.
Method 3: IVF transfer date
IVF pregnancies can be dated very precisely because the embryo’s age is known:
| Transfer | Due date |
|---|---|
| Day-3 embryo | Transfer date + 263 days |
| Day-5 embryo (blastocyst) | Transfer date + 261 days |
Why an early scan may change your date
Not everyone remembers their period date exactly, and ovulation doesn’t always happen on day 14. That’s why doctors use an early ultrasound to check. In the first trimester, babies grow at very similar rates, so measuring the baby’s length (the crown–rump length) gives an accurate age.
If the scan date and the period date differ by more than a few days, many guidelines recommend using the scan date. Once that early date is set, it generally stays fixed — later scans check growth rather than redate the pregnancy.
“Due date” really means “due window”
Only a small minority of babies are born on their due date. Doctors use these terms:
| Term | Weeks |
|---|---|
| Early term | 37 weeks 0 days – 38 weeks 6 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days |
| Post-term | 42 weeks and beyond |
Babies born before 37 weeks are called preterm. Your care team will talk to you about monitoring if you go past your due date.
Milestones worth knowing
- Around 6–7 weeks: a heartbeat is often visible on ultrasound.
- End of week 13: the first trimester ends.
- Around 18–22 weeks: the detailed anatomy (anomaly) scan.
- Week 28: the third trimester begins.
Find your dates
Our Pregnancy Due Date Calculator works with your last period (adjusted for your cycle length), a conception date, or an IVF transfer date. It shows your estimated due date, how many weeks pregnant you are, your trimester and the key milestone dates — a handy companion to the dates your doctor gives you.
Free toolTry the Pregnancy Due Date CalculatorFind your estimated due date, how far along you are and upcoming milestones.Frequently asked questions
Why is pregnancy counted from the last period and not conception?
Most people know when their last period started, but not the exact day they conceived. Counting from the period gives everyone the same reference point, which is why you are considered about two weeks pregnant at conception.
My scan date and period date don't match. Which is right?
If an early ultrasound differs from the period-based date by more than a few days, doctors usually go with the ultrasound. Follow the date your doctor confirms.
Can the due date change later in pregnancy?
Once a reliable early date is set, it normally isn't changed by later scans. Later scans measure growth, which varies more between babies.
For general information only — not medical advice, diagnosis or treatment. Always consult a qualified doctor, pharmacist or healthcare professional. Written by the Toolsopedia team. Spotted an error? Tell us and we’ll fix it.