Educational content, not medical advice. Clinical review of this page is in progress.
These two terms are often confused, but they measure pregnancy duration from two different starting points.
Gestational Age
Counted from the first day of your last menstrual period. This is the standard measurement used by doctors, calculators, and due date estimates.
Fetal Age (Conceptional Age)
Counted from the actual date of conception, which typically runs about 2 weeks behind gestational age.
Why Doctors Use Gestational Age
The exact date of conception is rarely known with certainty, while the last menstrual period is a more reliably reported reference point — making gestational age the practical, standardized choice.
A Simple Example
Suppose your last menstrual period began on January 1. Ovulation and conception typically happen about two weeks later, around January 15. On February 12, your gestational age is 6 weeks 0 days, counted from January 1, while the fetal age is about 4 weeks, counted from conception. Both numbers describe the same pregnancy; they simply start the clock at different moments.
Which Number Do Calculators and Charts Use
Nearly every prenatal tool uses gestational age: your due date, ultrasound reports, growth charts, screening windows and the “weeks pregnant” figure your provider quotes. A full-term pregnancy is about 40 weeks of gestational age, which is roughly 38 weeks from conception. You can see both numbers side by side in our Gestational Age Calculator.
Why the Two Numbers Differ by About Two Weeks
Conception cannot be observed directly, but the first day of your last period usually can be. Dating from that day gives everyone a common, easy-to-record starting point. In an average 28-day cycle, ovulation falls near day 14, which is why the two clocks differ by roughly two weeks. If your cycles are longer, shorter or irregular, the gap can vary, which is one reason an early ultrasound may adjust the date. Learn more in How Doctors Calculate Gestational Age.
Should You Worry About the Difference
No. The difference is a matter of counting convention, not a sign that anything is wrong. If you are ever unsure which number a chart or article uses, check whether it is measured from the last menstrual period or from conception, and ask your provider if it is unclear.
Related Reading
- How Doctors Calculate Gestational Age — LMP, ultrasound and adjustments
- Gestational Age FAQs — quick answers to common questions
- Traditional vs. IVF Dating — how the starting point changes
- When Gestational Age Gets Adjusted — why a due date can change
Also see our calculator FAQs, pregnancy FAQs and pregnancy terms glossary.
Frequently Asked Questions
If I am six weeks pregnant, is that gestational age or fetal age?
It is almost always gestational age, counted from the first day of your last menstrual period. The baby’s developmental, or fetal, age is about two weeks less.
Do baby size charts and pregnancy apps use gestational age?
Yes. Most charts, apps and calculators, including week-by-week guides, use gestational age.
Does the difference change my due date?
No. The due date is the same either way: about 40 weeks from your last period, or roughly 38 weeks from conception.
Related Resources
- Gestational Age Calculator
- How Doctors Calculate Gestational Age
- When and Why Gestational Age Gets Adjusted
- Pregnancy Milestones by Gestational Age
- Due Date Calculator
Sources and Further Reading
- ACOG: Methods for Estimating the Due Date
- ACOG: How Your Fetus Grows During Pregnancy
- MedlinePlus: Pregnancy
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN or midwife about your own pregnancy.