Gestational Age Calculator
Gestational age is reported as completed weeks plus days, counted from the first day of the last menstrual period or from ultrasound dating.
Example: For a last period starting 1 January 2026 with a 28-day cycle, the estimated due date is 8 October 2026. On 1 March 2026 that pregnancy would be 8 weeks 3 days along, in trimester 1.
Enter a date to see your estimate.
Estimates only. Around 4% of babies are born on their estimated due date. Ultrasound dating supersedes last-period dating.
Find an OB-GYN near youHow this is calculated
Where an ultrasound measurement is supplied, the dating anchor is derived from the scan date minus the measured gestational age, and this supersedes last-period dating. Otherwise the last-period date is used directly, adjusted for cycle length.
This tool is for information only. It does not diagnose, treat, or replace advice from a licensed clinician. Reviewed against ACOG guidance.
Frequently Asked Questions
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period, while fetal age is counted from conception. Gestational age is therefore the larger number, by the gap between your period and ovulation. Clinics, scan reports and this calculator all use gestational age.
How does an ultrasound work out gestational age?
The sonographer measures the baby and compares that measurement against expected size for each stage, then reports an age in weeks and days as of the scan date. This calculator takes that reported age and the scan date and works backwards to a dating anchor. That is how scan dating replaces last-period dating here.
My ultrasound age does not match my last period. Which one is right?
The ultrasound, in most cases, because scan dating supersedes last-period dating when the two disagree. Dating from your last period assumes ovulation happened on schedule, and that assumption is often wrong. Ask your OB-GYN which dating your chart uses so every later measurement stays consistent with it.
Why does an accurate gestational age matter?
Because nearly everything in prenatal care is scheduled by it: screening tests, growth checks and decisions about timing near the end of pregnancy. An age that is off can push a test outside the window where its result is useful. That is why dating is settled early rather than revisited late.
Can I work out gestational age without a scan?
Yes, from the first day of your last menstrual period, adjusted for your cycle length â that is the default method here. It is reasonable when your cycles are regular and you are sure of the date. It is least reliable with irregular cycles, which is exactly the situation where a dating scan helps most.
What if this does not match the weeks my clinic gave me?
Go with your clinic and ask your OB-GYN to explain the difference. They may be dating from a scan you have not entered here, or using a slightly different anchor, and your chart is the version that governs your care. Bring the dates you have and let them reconcile it.