How Due Dates Are Traditionally Calculated
This calculator estimates your due date using Naegele's rule, the standard method used in obstetrics for over 150 years: take the first day of your last menstrual period (LMP) and add 280 days, or 40 weeks. Pregnancy is conventionally dated from the LMP rather than the estimated conception date because most people know their period dates with far more certainty than their exact conception date, even though conception itself typically occurs about two weeks after the LMP in a standard cycle.
Adjusting for Your Actual Cycle Length
Naegele's rule was originally built around a textbook 28-day cycle, but not everyone ovulates on day 14. This calculator adjusts the standard 280-day estimate based on your actual average cycle length: a longer cycle shifts ovulation (and therefore your due date) later, while a shorter cycle shifts it earlier. This adjustment gives a more personalized estimate than the flat 40-week rule alone, particularly useful for anyone whose cycles are consistently longer or shorter than 28 days.
Why Due Dates Are Estimates, Not Guarantees
Only a small percentage of babies are actually born on their exact calculated due date — most arrive within a window of about two weeks before to two weeks after, and that's entirely normal. Due dates are best understood as the midpoint of a reasonable delivery window rather than a fixed appointment, which is why healthcare providers usually talk in terms of "around" a due date and monitor progress with ultrasounds that can refine the estimate as pregnancy progresses, particularly a dating scan performed in the first trimester.
Understanding Gestational Age
Gestational age, the number shown here as "current gestational age," is measured in weeks and days starting from the first day of your last period, not from the date of conception — which means in the first two weeks of "pregnancy" by this dating convention, conception typically hasn't even occurred yet. This can feel counterintuitive, but it's the universal standard used by every healthcare provider, which is exactly why this calculator uses the same convention: it keeps your dates consistent with what your doctor or midwife will reference at every appointment.
When to Confirm With a Healthcare Provider
This calculator provides a useful planning estimate, but it is not a substitute for medical care. An early ultrasound dating scan, typically performed between 8 and 13 weeks, is considered the most accurate way to confirm gestational age and due date, especially for anyone with irregular cycles where LMP-based dating is less reliable. If you believe you're pregnant or are planning a pregnancy, our Ovulation Calculator can help estimate your fertile window, but always follow up with a qualified healthcare provider for personalized medical guidance.
A Quick Tip on Tracking Your Cycle
If you're unsure of your exact average cycle length, review your last three to six cycles using a period-tracking app or calendar notes before using this calculator, since a more accurate cycle length input directly improves the precision of your estimated due date.
Frequently Asked Questions
How accurate is a due date calculated from LMP?
It's a solid estimate, but only a small fraction of babies arrive on the exact calculated date; a two-week window either side is considered entirely normal.
Why does cycle length affect the due date?
A longer or shorter cycle shifts when ovulation actually occurs relative to the standard 28-day assumption, which shifts the whole pregnancy timeline earlier or later.
Is LMP dating or ultrasound dating more accurate?
An early first-trimester ultrasound is generally considered more accurate than LMP dating alone, especially for people with irregular cycles.
What does 'gestational age' mean?
It's the medical convention of counting pregnancy duration from the first day of the last period, not from the actual date of conception, which occurs about two weeks later.