Pregnancy Due Date Calculator
Estimate your due date from the first day of your last menstrual period.
What this calculator does
Estimates your baby's due date from the first day of your last menstrual period (LMP), using the standard clinical method (Naegele's rule) that most healthcare providers use as a starting reference point.
Who this is for
Anyone in early pregnancy wanting a quick reference due date before or alongside their first prenatal appointment, or checking how far along they are and which trimester they're in.
How this calculator works
Uses Naegele's rule: adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle. An ultrasound dating scan is more accurate, especially for irregular cycles.
Worked example
If the first day of your last period was March 1: adding 280 days lands on December 6 as the estimated due date. A simpler mental shortcut some clinicians use: subtract 3 months from the LMP date, then add 7 days — March 1 minus 3 months is December 1, plus 7 days is December 8, which comes out a couple of days different from the precise 280-day calculation due to how calendar months vary in length. The 280-day method is the more accurate of the two.
Why Due Dates Are Estimates, Not Predictions
Naegele's rule was developed in the early 1800s based on a fixed 28-day cycle with ovulation on day 14 — assumptions that don't hold for every person. Actual cycle length varies significantly between individuals, and even for the same person cycle to cycle, which is the main reason LMP-based due dates carry meaningful uncertainty. Research consistently shows first-time mothers tend to deliver slightly later than the LMP-based estimate on average, while subsequent pregnancies trend slightly earlier — a pattern obstetric guidelines account for but that a simple LMP calculation alone doesn't capture.
Why the Dating Ultrasound Often Overrides LMP
A first-trimester ultrasound measures the fetus directly (typically via crown-rump length between 8-13 weeks) rather than relying on a remembered LMP date, which is why clinicians generally use ultrasound dating instead of LMP dating whenever the two disagree by more than about 5-7 days. If your cycles are irregular, or you're unsure of your exact LMP date, the ultrasound-based date is considerably more reliable than the LMP calculation this tool provides.
Where you are in the pregnancy
Run the calculator above to see your progress through the three trimesters.
Common mistakes
- Assuming a 28-day cycle when yours is different. Naegele's rule assumes ovulation on day 14 of a 28-day cycle — if your cycle runs longer or shorter, your actual due date may shift by several days.
- Treating the due date as precise. Fewer than 5% of babies are born on their exact estimated date — a range of a couple weeks either side is completely normal.
- Skipping the dating ultrasound. An early ultrasound (typically 8-13 weeks) generally gives a more accurate due date than LMP-based calculation alone, especially with irregular cycles.
- Confusing gestational age with fetal age. Pregnancy is conventionally dated from the LMP, roughly 2 weeks before conception actually occurs — so "6 weeks pregnant" means about 4 weeks since conception, not 6.
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Frequently Asked Questions
Why is pregnancy dated from my last period instead of conception?
Conception date is rarely known precisely, while the first day of the last period is a clear, memorable reference point. Since ovulation typically occurs around 2 weeks after the period starts, "gestational age" runs about 2 weeks ahead of actual embryonic/fetal age.
How accurate is this due date?
It's a general estimate based on a standard 28-day cycle. Fewer than 5% of babies are born on their exact calculated due date — a range of about two weeks either side is typical, and an ultrasound dating scan is generally more precise.
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle with ovulation around day 14. If your cycle is consistently longer or shorter, your actual due date may differ from this estimate — an ultrasound dating scan accounts for this more accurately.
When does each trimester start?
The first trimester runs from conception through week 13, the second from week 14 through week 27, and the third from week 28 until birth (around week 40).
Should I trust my LMP date or the ultrasound date?
Clinicians generally favor the ultrasound date whenever it disagrees with the LMP-based estimate by more than about 5-7 days, since the ultrasound measures the fetus directly rather than relying on a remembered period date.
Do first pregnancies tend to go past the due date?
On average, yes slightly — research shows first-time mothers tend to deliver a bit later than the LMP-based estimate, while subsequent pregnancies trend slightly earlier, though individual variation is still substantial either way.