Why does my LMP not match my memory?

Ultrasound dating in the first trimester is often more accurate than LMP recall, especially with irregular cycles or late ovulation.

What is the 280-day rule?

Standard pregnancy dating adds 280 days (40 weeks) to LMP. This calculator reverses that relationship.

Calculate your last menstrual period from a known due date.

Due Date

Last Menstrual Period

Wednesday, Oct 29, 2025

Conception (est.)

Wednesday, Nov 12, 2025

How to use this calculator

Frequently Asked Questions

How to Use

What it calculates

Works backward from a known due date to estimate your last menstrual period and conception date.

When to use it

Use when your doctor gave you an ultrasound-confirmed due date and you want to know your estimated LMP.

Example

Due date October 5 → LMP around December 29 of the prior year.

Explore More

Find your estimated conception date from due date or last menstrual period.

Calculate pregnancy weeks and days from your last menstrual period.

Estimate your implantation window from ovulation or last menstrual period.

Calculate your estimated due date from embryo transfer day (3-day or 5-day).

Calculate your next period date and cycle info from your last menstrual period.

Estimate fertile and safe days using the calendar method (educational only).