What this tool does
Estimate a pregnancy due date from the first day of the last menstrual period and the usual cycle length.
How to use it
- Enter the first day of the last menstrual period.
- Choose the usual cycle length between 21 and 45 days.
- Review the estimated due date and discuss clinical dating with a maternity-care professional.
Formula and method
The standard estimate adds 280 days to the first day of the last menstrual period for a 28-day cycle. This tool shifts the date by the difference between the entered cycle length and 28 days and estimates conception around day 14 with the same adjustment.
Worked example
A last menstrual period beginning 1 January 2026 with a 28-day cycle gives 8 October 2026. A 30-day cycle shifts the estimate to 10 October.
Common uses
- Preparing an early pregnancy timeline
- Understanding the 40-week calendar estimate
- Taking an initial date to a prenatal appointment
Limitations
Cycle timing and ovulation vary, and the last-period date may be uncertain. A due date is not a prediction of the exact birth day. Early ultrasound and clinical information can confirm or revise obstetric dating.
FAQ
Does a 40-week estimate mean birth will happen that day?
No. It is a reference date for pregnancy care, and normal births occur before or after it.
What if my cycles are irregular?
The cycle adjustment may be unreliable. Ask a maternity-care professional about dating based on clinical history and ultrasound.