Estimated Due Date Calculator (Naegele's Rule)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/3/2026
The estimated due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period — this is Naegele's rule. For a last period starting January 1st, the estimated due date falls around October 8th.
Explanation
Naegele's rule, named after the German obstetrician who popularized it in the 19th century, remains the reference method for a first due-date estimate: it adds 280 days (40 weeks of amenorrhea) to the first day of the last menstrual period, assuming a 28-day cycle with ovulation on day 14. This calculator automatically adjusts the estimate if your usual cycle is longer or shorter than 28 days, by adding or subtracting the difference — a longer cycle shifts ovulation (and therefore conception) later in the cycle. This estimate remains statistical: only 4 to 5% of births happen exactly on the calculated date, with the large majority falling within the two weeks surrounding it. An early ultrasound (first-trimester dating scan) usually refines this estimate and takes precedence over the last-period calculation if they differ — this page never replaces real medical follow-up. For your exact age rather than a pregnancy term, see our age calculator, or our BMI calculator for another common health indicator.
Example: last period on January 1, 2026, 28-day cycle
Inputs
Last menstrual period: January 1, 2026. Cycle length: 28 days (standard).
Calculation
Estimated due date = January 1, 2026 + 280 days = October 8, 2026.
Result
The estimated term falls around October 8, 2026.
Frequently asked questions
Why use the last menstrual period date instead of the conception date?
The exact conception date is rarely known with certainty, whereas the last menstrual period date is a reference point most women can pinpoint precisely. Pregnancy is therefore conventionally dated in weeks of amenorrhea (counted from the last period), rather than in actual weeks of pregnancy (which start about two weeks later, at ovulation).
What if my cycle is irregular?
This calculator assumes a regular cycle of a given length; for very irregular cycles, the last-period estimate becomes less reliable. A first-trimester dating ultrasound, which directly measures the size of the embryo, then generally gives a more accurate estimate than a calculation based on the last period.
Does this estimate replace medical follow-up?
No. This calculator gives a first informative estimate, not a diagnosis or pregnancy monitoring. Only a healthcare professional (midwife, obstetrician) can confirm and refine this term, notably via ultrasound, and provide the medical follow-up needed throughout the pregnancy.