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Pregnancy Due Date Calculator
Estimate your due date from your last period or conception date, see how far along you are right now, and which trimester you're in — no account, nothing stored.
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Last period start date
Conception date
Estimated Due Date
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Enter a date above to calculate
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Weeks pregnant
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Trimester
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Days to go
Conception40 weeks
Trimester timeline
Trimester
Weeks
Date range
Pregnancy is typically counted as 40 weeks from the first day of your last period — about 2 weeks before conception actually happens. This is the standard medical convention, not an error.
This is an estimate, not a diagnosis. Only about 5% of babies are born on their exact due date — most arrive within 2 weeks before or after. Your OB/midwife will refine this estimate using an early ultrasound, which is generally more accurate than date-based calculations alone. Always follow your healthcare provider's guidance over this tool.
Quick answers
How accurate is the last-period method?
It assumes a textbook 28-day cycle with ovulation on day 14 — the same assumption used in the standard "Naegele's Rule" formula (due date = last period start + 280 days). If your cycles are longer or shorter than 28 days, your actual due date may shift by several days in either direction. An early ultrasound (typically 8–12 weeks) gives a more precise estimate.
Why 40 weeks if conception happens around week 2?
Medical convention counts pregnancy from the last menstrual period rather than the conception date, mostly because the LMP date is something people usually know for certain, while the exact conception date often isn't. So "40 weeks pregnant" really means about 38 weeks since conception.
What happens after my due date passes?
Most pregnancies that go past the due date are still perfectly normal — babies born anywhere from 37 to 42 weeks are considered full-term or later. Your provider will typically monitor more closely if you go past 40–41 weeks and discuss options like induction if needed. This is a conversation for your provider, not something to self-manage from a due date estimate.
How to use this calculator
It applies Naegele's rule: 280 days from the first day of your last period, or 266 days from a known conception date. Both are estimates of the same thing.
Use the first day of your last period, not the last day. The convention counts from the start of bleeding.
If you know the conception date, switch modes and enter that instead. It is the more direct input when it is known.
Read the due date, then the current gestational age and trimester underneath it.
Treat the date as the centre of a window. Most births do not land on it.
If an early ultrasound gives a different date, that measurement generally supersedes the calendar estimate.
A worked example: a last period on February 10
February 10, 2026 plus 280 days is November 17, 2026. Counted the other way, that is 40 weeks from the start of the last period.
The conception date for the same pregnancy works out to about November 3 when 266 days are added, which is 14 days after the period start — the arithmetic behind the two modes agreeing with each other.
From the due date, the page derives gestational age. On, say, September 1, 2026, that pregnancy would be about 29 weeks, which puts it in the third trimester, since this page starts the third trimester at 28 weeks. The first trimester runs through 13 weeks and 6 days, and the second through 27 weeks and 6 days.
What your results mean
The due date is a statistical centre, not an appointment. Only a small share of births occur on the estimated date, and a normal range extends a couple of weeks either side of it. A date two weeks later is not a late calculator; it is the same estimate under a different convention.
Gestational age is counted from the last period, not from conception, which is why a pregnancy is described as 40 weeks long when conception happened about two weeks into that count. The convention exists so that everyone is measuring from the same starting point.
Early ultrasound is the more accurate dating method when it is available, because it measures the embryo rather than inferring from a remembered date. Clinical practice generally sets the date once and does not keep revising it.
This is not prenatal care and it cannot tell you anything about the pregnancy itself. It computes a date from a date you supplied, and it is only as reliable as that input.
Frequently asked questions
Why 280 days and not nine months?
Because 40 weeks is 280 days, and the count starts about two weeks before conception. Nine calendar months is a rough conversion of the same estimate.
How accurate is the due date?
As a centre of a distribution, reasonably. As a prediction for one birth, not very. Only about one birth in twenty occurs on the estimated date, which is why clinicians talk about a window.
What if my cycle is longer than 28 days?
Then ovulation may arrive later and the calendar estimate can run early. Ultrasound dating handles this better than a fixed rule, and it is the usual way a long-cycle pregnancy gets a corrected date.
Can I use the conception date instead?
Yes. It adds 266 days, which is the same estimate expressed from the other end. Use whichever date you are more confident about.
When does each trimester start?
On this page, the first trimester runs through 13 weeks and 6 days, the second through 27 weeks and 6 days, and the third begins at 28 weeks. Different sources draw the boundaries slightly differently.
Is my information stored?
No. The calculation runs in your browser and nothing is saved or sent anywhere.
Sources & references
280 days from the last menstrual period is the standard estimate ACOG describes. Ultrasound dating is a clinical step this page cannot perform.