Pregnancy Formula Explained

This reference explains the Pregnancy formula used by our free Pregnancy Calculator. Use it to understand variables, units, and common mistakes before you calculate.

The formula

Weeks pregnant from LMP; trimester by week count

Variables and units

• LMP — First day of last menstrual period (date)
• cycle length — Average menstrual cycle (days (default 28))

Worked example

Inputs: LMP January 1, 2026; today July 31, 2026

1. Days since LMP = 212 days
2. Weeks pregnant = 212 ÷ 7 = 30 weeks 2 days
3. Trimester: third trimester (weeks 28–40)

Result: Approximately 30 weeks pregnant, third trimester

When to use this formula

Use this formula when you need a quick pregnancy estimate with values you already know. Our Pregnancy Calculator applies the same relationship automatically so you can verify handwritten work.

Common mistakes

• Using conception date instead of LMP without adjusting
• Ignoring irregular cycles when estimating
• Confusing gestational age with fetal age (fetal age is ~2 weeks less)

Frequently asked questions