What is the Difference Between Gestational Age and Fetal Age?

The main difference between gestational age and fetal age lies in how they are calculated and their purpose:

• Gestational Age: This is the age of the pregnancy measured from the first day of the last menstrual period (LMP). It is used to describe how far along a pregnancy is and is commonly used to measure the length of a pregnancy in weeks. Gestational age is divided into three trimesters: the first trimester (weeks 1 to 13), the second trimester (weeks 14 to 27), and the third trimester (weeks 28 to delivery).
• Fetal Age: This is the actual age of the growing baby, calculated from the date of conception, which occurs during ovulation. Since the date of conception is generally unknown for humans, fetal age is considered less precise than gestational age. Fetal age is about two weeks behind gestational age. Fetal development is the term used to describe the succession of biological events that occur in the fetus during pregnancy.

In summary, gestational age is based on the mother's biology and is used to measure the length of a pregnancy, while fetal age represents the actual age of the fetus and is calculated from the date of conception. Due to the difficulty in determining the exact date of conception, gestational age is the more common and precise way to measure pregnancy.

Comparative Table: Gestational Age vs Fetal Age

The difference between gestational age and fetal age can be confusing, but they are essentially the same concept. Both gestational age and fetal age refer to the age of the fetus during pregnancy, and they are typically assessed using ultrasound measurements. Here is a table comparing the two terms:

Ultrasound is the most accurate method for estimating gestational or fetal age, particularly in the first trimester (up to and including 13 6/7 weeks of gestation). In the second trimester, gestational age assessment is based on a composite of fetal biometric measurements for an accuracy of 7–10 days.