Pregnancy Due Date Calculator

What does this Pregnancy Due Date Calculator do?

This calculator estimates your baby's due date and current week of pregnancy from the first day of your last menstrual period (LMP), using the standard obstetric convention of a 280-day (40-week) pregnancy.

The formula

Due date = LMP + 280 days (Naegele's rule). Current week = number of days since LMP ÷ 7, rounded down.

Step-by-step example

If your last period started on January 1st, adding 280 days lands the estimated due date around October 8th. If today is 70 days after that LMP date, you're at day 70 ÷ 7 = week 10 of pregnancy.

Tips & common mistakes

• Naegele's rule assumes a standard 28-day cycle — if your cycle is meaningfully longer or shorter, your actual due date may differ from this estimate.
• An ultrasound dating scan, typically done early in pregnancy, is generally considered more accurate than LMP-based calculation alone — use this calculator as a starting estimate, not a replacement for medical dating.
• "Weeks pregnant" counts from the LMP, not from conception — conception typically occurs about two weeks after the LMP, which is why week 2 is often called "week 0" of actual fetal development in some contexts.

Frequently asked questions