Gestational Age Calculator
A last menstrual period of January 1 puts you at 27 weeks, 4 days by July 13. This gestational age calculator finds how far along a pregnancy is right now, not just the due date. Enter the first day of the last menstrual period and a reference date to see the current week, trimester, and percent through pregnancy.
Quick answer
Gestational age counts from the first day of the last menstrual period (LMP), not from conception.
What this tells you
- •Gestational age counts from the first day of the last menstrual period (LMP), not from conception.
- •Age-so-far is shown in completed weeks plus extra days, the same format used at prenatal visits.
- •Trimester and percent complete are based on a standard 280-day, 40-week pregnancy.
How to Use
- 1Enter the first day of the last menstrual period.
- 2Enter the reference date, usually today.
- 3Calculate to see the current gestational age, trimester, and percent through pregnancy.
How It Works
Formula
Gestational Age = Reference Date minus LMP DateThe days between the LMP date and the reference date are converted to completed weeks plus remaining days, using standard LMP-based dating.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Mid-pregnancy check
Common mistakes
- Confusing LMP-based dating with conception-based dating, which runs about 2 weeks earlier
- Using an uncertain or estimated LMP date
- Treating trimester cutoffs as exact medical boundaries
Limitations
This estimate assumes a standard 280-day, 40-week pregnancy dated from the first day of the last menstrual period. It does not account for irregular cycles, uncertain LMP recall, or ultrasound-based dating adjustments a provider may use, all of which can shift the actual gestational age by a week or more.