Gestational Age vs. Fetal Age
It's the #1 confusion in early pregnancy: Why does the doctor say I'm 4 weeks pregnant when I only conceived 2 weeks ago?
Gestational Age
Used by doctors and calculators. It counts from the First Day of your Last Period. It includes the ~2 weeks before you actually ovulated and conceived.
Fetal Age
The actual age of the embryo. This starts from the moment of conception (fertilization). It is typically 2 weeks less than your Gestational Age.
Why Doctors Change Your Dates (Redating)
You might walk in thinking you are 9 weeks based on your period, but leave with a new due date saying you are only 8 weeks. This is called Clinical Redating.
In the first trimester (up to 13 weeks), doctors measure the baby's Crown-Rump Length (CRL) via ultrasound. If this measurement differs from your LMP prediction by more than 5-7 days, the ultrasound date becomes your new, official "Medical Due Date" because early growth is extremely uniform across all pregnancies.
Frequently Asked Questions
My app says 8 weeks, but ultrasound says 7 weeks. Which is right?
What is the 'Rule of 10'?
Can I use this for IVF?
Tools & Data Verified by the EverydayCalculators Medical Research Team.
Last updated: January 2026.