This gives a general estimate only — roughly 1 in 20 babies actually arrives on their exact due date. Your OB or midwife's estimate, especially if based on an early ultrasound, is more precise than any calculator.
How this is calculated
The most common method (Naegele's rule) adds 280 days — 40 weeks — to the first day of your last menstrual period, assuming a fairly typical 28-day cycle. If you're calculating from a known conception date instead, we add 266 days (38 weeks), since conception itself usually happens about two weeks into a cycle.
Why your actual due date may differ
Cycle length varies a lot between people, and an early ultrasound (usually in the first trimester) is generally considered the most accurate way to date a pregnancy, since it measures the embryo or fetus directly rather than estimating from a remembered date. Only about 5% of babies are born on their exact estimated due date, most arrive within about two weeks before or after it.
The method behind the math: Naegele's rule
This calculator uses Naegele's rule, the standard method taught in obstetrics and used by most providers as a starting estimate. It assumes a 28-day cycle with ovulation around day 14, and adds 280 days (40 weeks) to the first day of your last menstrual period (LMP) to land on an estimated due date. If you enter a conception date instead, we add 266 days (38 weeks), since conception itself typically happens about two weeks into a cycle, and that shortcuts past the two weeks Naegele's rule otherwise counts before ovulation.
How accurate is it, really
Naegele's rule is a reasonable starting estimate, but it leans on an assumption, a textbook 28-day cycle, that does not describe everyone. If your cycles run shorter or longer than 28 days, or are irregular, your actual ovulation date could fall well before or after day 14, which shifts the true due date away from what a calculator based only on LMP will show. This is exactly why an early ultrasound, usually done in the first trimester, is considered more accurate for dating a pregnancy: it measures the size of the embryo or fetus directly, rather than working backward from a remembered date. According to ACOG, once an early ultrasound has established a due date, that estimate generally should not be changed based on later ultrasounds, since fetal size becomes a less reliable dating tool as pregnancy progresses.
How to use your estimated due date
Treat the date this tool gives you as the center of a range, not a fixed appointment. It is genuinely useful for figuring out which trimester you are in, roughly how many weeks along you are for tracking milestones, and when to expect key prenatal tests and appointments. It is not useful as a precise prediction of when labor will start. Full-term is generally considered 39 to 40 weeks, and normal, healthy births happen anywhere from about 37 to 42 weeks.
A quick checklist once you have an estimated due date
- Schedule your first prenatal appointment, ideally around 8 weeks
- Ask your provider to confirm dating with an early ultrasound if you have not had one
- Use the estimate to plan around key screening windows (first trimester screening, anatomy scan, glucose test)
- Start a prenatal vitamin with folic acid if you have not already
- Remember the date can shift slightly after your first ultrasound, and that is normal
Frequently asked questions
Is a due date calculator as accurate as an ultrasound?
No. A calculator based on your last period assumes a standard 28-day cycle, which does not apply to everyone. An early ultrasound measures the embryo or fetus directly and is generally considered more accurate for dating a pregnancy.
Why did my due date change after an ultrasound?
If your cycle is longer or shorter than 28 days, or ovulation happened earlier or later than typical, the ultrasound measurement may point to a different gestational age than a calculation based only on your last period. Providers generally use the earliest ultrasound's estimate going forward.
What percentage of babies are born on their due date?
Only about 5% are born on their exact estimated due date. Most babies arrive within roughly two weeks before or after it, which is why the date is best treated as an estimate rather than a deadline.
For more on how due dates are estimated and confirmed, see ACOG's guidance on pregnancy dating.