About the Pregnancy Due Date Calculator
Most due dates are estimated from the first day of your last menstrual period (LMP). This calculator applies Naegele’s rule, adjusts for your cycle length, and shows how far along you are today.
How to use the Pregnancy Due Date Calculator
- Enter the first day of your last period.
- Adjust your usual cycle length if it is not 28 days.
- See your estimated due date, current week and trimester.
- Confirm the date with your doctor; an early ultrasound is more accurate.
Formula used
Due date = first day of last period + 280 days + (cycle length − 28 days). Estimated conception ≈ LMP + 14 days + (cycle length − 28).
Frequently asked questions
How accurate is a due date?
Only about 1 in 20 babies arrive on the exact due date. Most are born within two weeks either side.
What if my periods are irregular?
LMP-based dates are less reliable with irregular cycles. Your doctor will usually date the pregnancy with an early ultrasound.
When do trimesters start?
The first trimester is weeks 1–13, the second weeks 14–27 and the third from week 28 until birth.
Last reviewed: November 7, 2026