How Is My Due Date Calculated?
How a due date is worked out from your last period using Naegele's rule and the 280-day count, why a dating scan can move it, and why it is an estimate.
Editorial draft: not yet reviewed by a clinician.
A due date is 280 days, or 40 weeks, from the first day of your last menstrual period. The quick way to work it out by hand is Naegele’s rule: first day of the last period, plus seven days, minus three months, plus a year. A first-trimester dating scan measures the pregnancy directly and may replace that date with a slightly different one.
The 280-day count
Forty weeks of 7 days is 280 days, and that is the whole calculation. It is counted from the first day of your last period rather than from conception, which is why you are already described as around four weeks pregnant when a test turns positive.
Counting this way rests on two assumptions: that your cycle is roughly 28 days, and that ovulation happened around the middle of it. For plenty of people neither is true, which is the main reason a scan-based date can differ.
Naegele’s rule, done by hand
The rule is the arithmetic shortcut that gets you to the same place without counting 280 days on a calendar.
- Start with the first day of your last menstrual period.
- Add 7 days.
- Subtract 3 months.
- Add 1 year.
| Step | Example |
|---|---|
| First day of last period | 10 January 2026 |
| Add 7 days | 17 January 2026 |
| Subtract 3 months | 17 October 2025 |
| Add 1 year | 17 October 2026 |
Because calendar months are not all the same length, this shortcut can land a day or two away from a strict 280-day count. Neither version is more real than the other. They are both estimates of the middle of a range.
Why a scan can move the date
A dating scan measures the baby, usually the crown-rump length, and estimates how far along the pregnancy is from that measurement. NHS guidance describes this scan as usually offered between 10 and 14 weeks. Early measurements are useful for dating precisely because babies grow at very similar rates in the first trimester, so the size is a reliable clue to age.
If the scan gives a different date from the one calculated from your period, your team will normally adopt the scan date and use it for the rest of your care, including when appointments and later scans are offered. This is routine. It is not evidence that anything is wrong, and it is not a statement about when you conceived.
Dating is more accurate the earlier the scan happens, which is why a date set at 12 weeks is not usually revised by a growth scan at 34 weeks. By then, measurements tell you about growth rather than about age.
How your week number works
Your gestational age is written as two numbers, such as 12+3, meaning twelve completed weeks and three days. You are “in your thirteenth week” when you are 12+0, which is a common source of confusion. Trackers, including a week-by-week guide, normally label content by completed weeks, so week 12 content covers 12+0 to 12+6.
What the due date is not
- It is not a deadline. The NHS describes a full-term birth as one between 37 and 42 weeks, so the useful mental model is a five-week window with your due date near the middle.
- It is not a prediction of labour. Nothing about the date tells you whether you will go early or late.
- It is not fixed by you. If you recalculate it from an app and get a different answer from your notes, go with your notes and ask about the difference at your next appointment.
- It is not a reason to book things tightly. Arranging leave, help and visitors around a window rather than a date saves a lot of frustration.
Situations where the standard calculation does not apply
- Irregular, long or short cycles. The 28-day assumption fails, and dating by scan is more reliable.
- You do not know the date of your last period. Very common, and not a problem. Say so.
- Bleeding that may not have been a true period. Worth mentioning, because it can shift the count.
- Pregnancy after fertility treatment. Dating is usually based on the known date of the procedure rather than a period.
- Twins or more. Dating is still done by scan, but the rest of your care plan, including when birth is likely, is different and specific to you.
Why two calculators can give you different dates
If you enter the same last period into two apps and get answers a day or two apart, neither of them is broken. Some count a strict 280 days, some apply Naegele’s rule, and some adjust for a cycle length you have entered. A difference of a couple of days has no practical meaning for your care. The date in your maternity notes is the one your appointments and scans are planned around, so if an app disagrees with your notes by more than a few days, raise it at your next appointment rather than trying to settle it with arithmetic.
Keeping the date and the count in one place in PreBaby
PreBaby counts your week and day from either the due date or the first day of your last period, whichever you have, and shows the matching week’s content for weeks 4 to 42. If a dating scan moves your date, you can change it and the whole count follows, so you are not doing calendar arithmetic every week. If you have invited a partner as a follower, they see the same week you do.
Sources
This article is general information, not medical advice. Anything that worries you is a reason to contact your own midwife or doctor.
Questions and answers
How is a due date worked out from my last period?
The standard method counts 280 days, or 40 weeks, from the first day of your last menstrual period. The shortcut used to do it by hand is Naegele's rule: take the first day of your last period, add seven days, subtract three months, then add a year. So a last period starting on 10 January 2026 gives an estimated due date of 17 October 2026. Because calendar months differ in length, the shortcut can land a day or two away from a strict 280-day count. Both are estimates, and a first-trimester scan may replace them with a slightly different date.
Why did my dating scan change my due date?
Because a scan measures the pregnancy itself rather than relying on the date of a period and an assumed cycle. NHS guidance describes a dating scan usually offered between 10 and 14 weeks, which measures the baby and estimates gestational age from that measurement. Early measurements are a good guide to age because babies grow at very similar rates in the first trimester. If the scan date differs from the date calculated from your last period, your team will normally work from the scan. That is routine, it does not mean anything was wrong, and it does not mean you conceived on a different date than you think.
Does a due date mean the baby will arrive that day?
No. The due date is the midpoint of a range, not a deadline, and only a small minority of babies arrive exactly on it. The NHS describes a full-term birth as one happening between 37 and 42 weeks, which is a window of more than a month. Your own team will discuss what happens if you go past your due date, including what monitoring or induction they offer and when, and that varies between services and between pregnancies. Planning around the due date as though it were a booked appointment is the main reason it causes disappointment.
What if my cycle is irregular or I do not know my last period?
This is common, and it is exactly the situation an early scan is good at. If your cycles are long, short or irregular, the assumption behind the calculation, a roughly 28-day cycle with ovulation around the middle, does not hold, so a date from your last period can be off by a week or more. Tell your midwife or doctor that your cycle is irregular or that you are unsure of the date, and they can date the pregnancy by scan measurement instead. If the pregnancy followed fertility treatment, dating is usually based on the known date of the procedure.
Sources
This is general information, not medical advice. It does not diagnose or treat, and it does not replace your own midwife or doctor. Anything that worries you is a reason to contact them.