How to Calculate Weeks Pregnant

Gestational age is counted from the first day of your last menstrual period (LMP), not from conception. This standard adds about two weeks before actual fertilization.

Step-by-step

1. Identify the first day of your last menstrual period (LMP).
2. Count the number of days from LMP to today.
3. Divide by 7 to get weeks and remaining days.
4. Determine trimester: first (weeks 1–12), second (13–27), third (28–40).
5. If cycles are irregular or LMP is uncertain, use an early ultrasound (before 14 weeks) for dating.
6. Enter LMP into our Pregnancy Calculator for instant weeks, days, and trimester.

Tips and common mistakes

• IVF pregnancies date from embryo transfer (add 14 days for day-5 blastocyst)
• Ultrasound dating overrides LMP when discrepancy exceeds 5–7 days in first trimester

Open the Pregnancy Calculator to apply these steps with your own numbers instantly.

Frequently asked questions