Pregnancy Due Date Estimator
Estimate due date from last menstrual period using Naegele's rule.
Overview
A pregnancy due-date estimator gives an approximate delivery date from the first day of the last menstrual period (LMP). It is the standard first calculation made at an initial prenatal visit and is used to schedule scans, lab tests, and antenatal appointments, even though only about 4% of babies actually arrive on the predicted date.
The estimate is most accurate for people with regular 28-day cycles, where ovulation reliably falls around day 14. For longer or shorter cycles, an ultrasound-based date — usually the crown-rump length measurement between 8 and 13 weeks — overrides the LMP estimate because embryonic size in the first trimester is highly consistent across pregnancies.
How it works
Naegele's rule, named after the German obstetrician Franz Naegele, calculates the estimated date of delivery as LMP + 280 days, or equivalently LMP + 1 year − 3 months + 7 days. Two hundred and eighty days corresponds to 40 weeks counted from LMP, even though true gestation (from conception) averages about 38 weeks.
The tool also reports current gestational age in weeks and days, the date of conception (approximated as LMP + 14 days), and trimester boundaries (weeks 1–13, 14–27, and 28+). For cycles longer or shorter than 28 days, the calculator adjusts by adding or subtracting the difference between the user's cycle length and 28, since ovulation shifts with cycle length.
Examples
- LMP 1 January with a 28-day cycle: due date 8 October (1 Jan + 280 days).
- LMP 15 March 2026 with a 28-day cycle: due date 20 December 2026; today (18 May) is approximately week 9 + 2.
- LMP 1 February with a 35-day cycle: standard Naegele gives 8 November; cycle-length adjusted estimate is 15 November (added 7 days).
- LMP 10 June with a 26-day cycle: standard 17 March next year; adjusted 15 March (subtracted 2 days).
FAQ
How accurate is the LMP-based date?
Within about ±7 days for most regular cycles. Early ultrasound is more accurate and is the clinical standard.
Why is gestational age counted from LMP rather than conception?
LMP is a clear, observable event; conception is not. The convention of counting from LMP predates ovulation testing and is universal.
What if I do not remember my LMP?
Use a recent ultrasound's stated gestational age, or consult a clinician for a dating scan in the first trimester.
Does the due date change throughout pregnancy?
If early ultrasound dating differs from LMP dating by more than about a week, the ultrasound date is usually adopted as the official one.