Pregnancy Due Date Calculator

How this due date calculator works

Estimated due dates are commonly calculated from the first day of the last menstrual period (LMP) using Naegele’s rule: add 280 days (40 weeks). That convention assumes a 28-day cycle with ovulation around day 14. Many pregnancies are also dated by ultrasound, which can revise the estimate.

Why LMP is used

Conception timing is hard to pin exactly. LMP is a date most people can report, so clinics standardized on it. If cycles are irregular, longer, or shorter than 28 days, the calendar due date may be less accurate — another reason ultrasound dating matters.

Worked example

If LMP was March 1, adding 280 days lands near December 6 (exact calendar math depends on month lengths). The page also shows how many weeks pregnant you would be from today relative to that LMP, which is useful for understanding trimester language.

How to use it

1. Enter the first day of your last menstrual period.
2. Read the estimated due date and current gestational age estimate.
3. Bring the same LMP date to your clinician; follow their dating if ultrasound differs.

Frequently asked questions

What if I do not know my LMP exactly?

Give your clinician the best estimate and any cycle notes. Ultrasound dating may replace LMP-based dates when dates are uncertain.

Does a due date mean the baby arrives that day?

No. It is an estimate around which birth often occurs within a window of weeks. Only a clinician should interpret timing for your pregnancy.

Limitations

This tool does not diagnose pregnancy or predict complications. IVF and known ovulation dates use different dating methods.

Not a substitute for professional medical care.