Gestational Age Calculator
This gestational age calculator estimates how far along a pregnancy is and the estimated due date (EDD), either from the last menstrual period (Naegele's rule) or from an ultrasound scan. It reports weeks + days and the current trimester.
Calculator
LiveEnter values to see your result.
How to Use the Calculator
- Choose the LMP or ultrasound method.
- Enter the last menstrual period date, or the scan date and the gestational age measured at that scan.
- Read the current gestational age, estimated due date, and trimester.
What the Result Means
Gestational age is reported in completed weeks plus days — the standard obstetric format. The estimated due date (EDD) is the projected 40-week delivery date. Ultrasound dating is preferred when the LMP is uncertain or differs from a first-trimester scan by more than seven days.
When to Use This Calculator
- Dating a pregnancy from the LMP
- Dating from an ultrasound scan
- Finding the estimated due date (EDD) and trimester
Frequently Asked Questions
How is gestational age calculated from the LMP?
Gestational age is the number of days from the first day of the last menstrual period to today, divided by 7 for weeks. The due date is the LMP plus 280 days (Naegele's rule).
What is the difference between gestational age and EDD?
Gestational age is how far along the pregnancy is right now (in weeks and days). The estimated due date (EDD) is the projected date of delivery at 40 weeks.
Is LMP or ultrasound dating more accurate?
Ultrasound dating is preferred when the LMP is uncertain or when it differs from a first-trimester scan by more than 7 days, per ACOG guidance.
References & Guidelines
- ACOG / AIUM. Methods for Estimating the Due Date. Committee Opinion.