LMP calculator

Calculate LMP from Gestational Age

Gestational Age (Weeks):

Additional Days:

Reference Date (Today):

What is the Last Menstrual Period (LMP)?

The LMP is the first day of a woman's last menstrual period before she became pregnant. It's commonly used as a reference point to estimate the due date and track the progress of pregnancy.

What is Gestational Age (GA)?

Gestational Age refers to how far along a pregnancy is, measured in weeks and days from the LMP, not from the date of conception. For example, if a pregnancy is said to be 10 weeks and 4 days, it means it has been 10 weeks and 4 days since the LMP.

Why Calculate LMP from GA?

Sometimes, you know the gestational age (from an ultrasound, for example) but not the LMP. In such cases, you can work backward from today's date to find the estimated LMP.

📅 How to Calculate LMP from GA:

Step 1: Convert Gestational Age to Days

• Multiply the number of weeks by 7.
• Add the remaining days.
• Example: 12 weeks and 5 days → 12×7+5=8912 \times 7 + 5 = 8912×7+5=89 days.

Step 2: Subtract the Total Days from Today's Date

• Use a calendar or date calculator.
• Example: If today is April 9, 2025 and GA is 12 weeks and 5 days →
  April 9, 2025 − 89 days = January 10, 2025

This result (January 10, 2025) is the estimated LMP.

📝 Summary Formula:

LMP = Today's Date - Gestational Age (in days)