Overview
Gestational age is commonly counted from the first day of the last menstrual period and expressed in weeks and days. This tool estimates it from a chosen reference date, with trimester and progress, while clinical confirmation remains with the clinician.
Formula
The difference in days between the reference date and calculation date is converted into weeks and days. Conception adds 14 days, and ultrasound adds the gestational age at the scan.
How it works
Choose the method
Last period, conception date or ultrasound.
Enter dates
Reference date and calculation date, plus scan age if needed.
Review the estimate
Weeks, days, trimester and progress appear.
Worked examples
Last period on January 1
On March 1, gestational age is about 8 weeks and 3 days.
An eight-week scan
If the scan is on March 1 at eight weeks, two weeks later the age is 10 weeks.
Frequently asked questions
Is dating from the last period accurate?
It depends on cycle regularity. First-trimester ultrasound is often more accurate, and the clinician decides the official method.
Why does pregnancy start before conception?
The common count starts at the last period, about two weeks before conception, so gestational age is ahead of the conception date.
Does it work for multiple pregnancies?
It gives a general estimate, but multiple pregnancies need specialist follow-up and dates.
Does it set the due date?
No. Use the due date calculator for an estimate; the official date is set by the clinician.
This calculator is for educational and informational purposes only and is not a substitute for professional advice.