How is my due date calculated? The 280-day math explained
How is my due date calculated: Naegele's rule adds 280 days to your last period. Here is the full math, the 28-day-cycle assumption, and why it shifts.
Your due date is calculated by adding 280 days, or 40 weeks, to the first day of your last menstrual period. That formula is called Naegele’s rule, and it is what nearly every due date calculator and provider starts from. It assumes a 28-day cycle with ovulation around day 14, which is exactly why your real date can move once a different cycle length or an early ultrasound says otherwise.
This guide walks through the arithmetic, why the count starts from your period rather than conception, how ultrasound dating differs, and why almost no one delivers on the printed date. It is general education about how the estimate works, not medical advice about your own pregnancy.
The formula: last period plus 280 days
The whole calculation rests on one date and one number. The date is the first day of your last menstrual period, usually written LMP. The number is 280 days.
Estimated due date = first day of your last period + 280 days (40 weeks)
There is a well-known shortcut that gets you the same answer without counting 280 days on a calendar. Take the first day of your last period, count back three calendar months, then add one year and seven days. If your last period started on May 10, subtract three months to get February 10, then add a year and a week to land on February 17 of the following year. That February 17 is your estimated due date.
Both routes give the same result because they are the same rule written two ways. A calculator simply does the addition instantly, which is handy when the three-month subtraction crosses into a new year.
Why it starts from your last period, not conception
It seems backward to count pregnancy from a day when you were not yet pregnant, but there is a practical reason. Almost everyone knows the day their last period started. Very few people know the exact day they conceived, because ovulation and fertilization happen quietly inside a several-day fertile window.
So obstetrics uses the one date that is reliably known and visible: the first day of the last period. The trade-off is that the count includes roughly two weeks before conception even happened. That is normal and expected, and it is the reason you are considered “about 2 weeks pregnant” at the moment of conception. If that quirk is confusing, how many weeks pregnant am I unpacks the counting in full.
The 28-day-cycle assumption, and when it breaks
Naegele’s rule bakes in two assumptions: that your cycle is 28 days long, and that you ovulated on day 14. For a lot of people those are close enough. For plenty of others they are not.
If your cycles run longer than 28 days, you probably ovulate later than day 14, which means conception happened later than the formula assumes, and a pure last-period estimate can read a few days early. Shorter cycles push it the other way. This is not an error in the math; it is the math meeting a body that does not run on a textbook 28-day clock. If your cycles are irregular, the last-period estimate is looser still, which is one reason providers lean on an early scan.
You can see the same day-14 assumption at work in the free ovulation calculator, which estimates your fertile window from your cycle length. Feeding it your real cycle length rather than a generic 28 days is what makes any date built on it more honest.
Conception dating versus ultrasound dating
There are two other ways to date a pregnancy, and they answer slightly different questions.
- Conception dating counts from the day of fertilization instead of the last period. Because conception is normally about two weeks after the period starts, you add roughly 266 days (38 weeks) rather than 280. It is only useful when you actually know the conception date, for example after tracked ovulation or a fertility procedure.
- Ultrasound dating does not rely on your calendar at all. An early scan measures the physical size of the embryo, which grows at a steady, predictable rate in the first trimester. From that measurement the sonographer reads off how far along the pregnancy is and sets a due date directly.
Because a first-trimester scan measures growth rather than assuming a cycle, it is generally the more precise of the two, especially when your cycle is long, short, or irregular. When the scan date and the last-period date disagree by more than about a week, providers commonly redate the pregnancy to the scan. That is why your due date can change at your first appointment, and a small change is completely routine.
Why almost no one delivers on the exact date
Here is the part worth internalizing: the due date is a single day standing in for a range. Only about 4% of babies are born on their estimated date. The rest arrive in the weeks before or after, and a birth anywhere across the full-term window is normal.
Two things drive that spread. First, the dating itself carries some uncertainty, since it leans on cycle assumptions or a measurement with its own small margin. Second, labor simply does not start on a schedule; the biology that triggers it varies from person to person. Put together, they mean the estimate points at a midpoint, not a finish line. Treating it as a rough center rather than a promise saves a lot of last-week anxiety.
If you want the weeks between now and then in one calm place, PregnancyOS is a single offline HTML file that shows your current week out of 40 alongside appointments, symptoms, weight, and questions for your provider. It is bought once for $23, works on a laptop or phone, and everything you type stays in your browser on your own device, with no account and nothing uploaded. It is a planner, not medical care, and a paper calendar tracks the same weeks; the point is only that the count lives somewhere other than your head.
Find your own date
To turn all of this into a real date, enter the first day of your last period in the due date calculator. It applies the 280-day rule in a second and gives you the estimate to bring to your first appointment, where an early scan may fine-tune it. From there, learning how many weeks pregnant you are tells you what is happening right now. If you are not yet sure you are pregnant, early signs of pregnancy covers what commonly shows up, and if you are still in the two week wait before a positive test, the two week wait explains why the timing feels so slow.
This article is general education, not medical advice, and it cannot date your pregnancy or tell you when you will deliver. Your own OB or midwife sets and adjusts your due date based on your cycle and your ultrasounds.
Frequently asked questions
How is my due date calculated?
Your due date is estimated by adding 280 days (40 weeks) to the first day of your last menstrual period. This is called Naegele's rule, and it is what almost every due date calculator and clinician uses. A common shortcut is to take the first day of your last period, subtract three months, then add one year and seven days.
Can I calculate my due date from the conception date instead?
Yes, if you know the date. Because conception usually happens about two weeks after your period starts, you add roughly 266 days (38 weeks) to the conception date instead of 280. Most people do not know the exact day they conceived, which is why the standard method counts from the last period instead.
Does an ultrasound change my due date?
It can. An early ultrasound measures the size of the embryo, which grows at a predictable rate in the first trimester, so it dates the pregnancy directly rather than assuming a cycle length. If that measured date differs from your last-period date by more than about a week, providers often move the due date to match the scan.
What if my cycle is not 28 days?
Naegele's rule assumes a 28-day cycle with ovulation on day 14, so a longer or shorter cycle shifts your real timing. If you ovulate later than day 14, the last-period estimate tends to run a little early, and an ultrasound usually corrects it. Tell your provider your typical cycle length so the estimate can be adjusted.
Are babies usually born on their exact due date?
No. Only about 4% of babies arrive on the exact estimated date, and a full-term birth anywhere in the weeks around it is normal. The due date is a midpoint, not a deadline, so treat it as the center of a range rather than a fixed appointment.
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