Understanding the estimate
The timeline starts before conception
This calculator starts with the first day of the last menstrual period, often abbreviated LMP. That is the dating convention used here, not a claim that conception happened on that day.
Starting point
First day of the last period
Enter the first day, not the last day of bleeding.
Calendar estimate
Add 280 days for a 28-day cycle
For the fixed example 1 January 2026, this gives 8 October 2026.
Cycle adjustment
Add the difference from 28 days
A 30-day cycle adds two more days, giving 10 October in the same example. This is a simplified estimate.
Why a clinician’s date may differ
The 280-day convention assumes a 28-day cycle. Real ovulation timing can differ, and an uncertain period date makes a calendar estimate less reliable. The cycle-length adjustment cannot establish the actual date of ovulation or conception.
ACOG’s guidance on estimating the due date identifies first-trimester ultrasound as the most accurate way to establish or confirm gestational age. Use the date established with your maternity care team for clinical planning. Guidance checked September 2026.
Gestational age
The weeks and days above count calendar days since the entered LMP. Changing cycle length shifts the estimated due date, but does not shift this tool’s LMP-based gestational-age counter.
Progress and trimesters
The progress bar compares elapsed days with 280 days; it is not a measure of fetal development. The tool starts the second trimester at 14 weeks and the third at 28 weeks.
An estimate is not an appointment
The due date is a reference date, not a prediction of the exact day of birth. A “Due in” or “Overdue by” label describes the arithmetic difference from that estimate; it does not assess wellbeing or determine when care is needed.
This tool accepts a past or current LMP and a whole-number cycle length from 20 to 45 days. It hides results more than 45 weeks after the entered LMP. Those input limits are software limits, not a definition of a typical cycle or a clinical recommendation.
For irregular cycles, uncertain dates, or assisted reproduction, discuss dating with your care team rather than trying to force an input that fits this model. Your entries are processed in the browser and are not saved. Reload for a fresh current-date calculation if the page stays open overnight.