What is the Difference Between Gestational Age and Fetal Age

​​​​The main difference between gestational age and fetal age is that gestational age is the age of the pregnancy taken from the beginning of the woman’s last menstrual period (LMP). But, fetal age is the actual age of the growing baby, starting at the time of conception. Thus, gestational age includes an additional two weeks time where the woman was not pregnant.  

Gestational age and fetal age are the two time periods used to talk about the length of a pregnancy. More often, pregnancy refers to the gestational age rather than fetal age due to the accuracy of the measurement. However, their main function is to determine the due date of pregnancy. In general, gestational age is also known as the menstrual age, while fetal age is also known as the conceptual age. 

Key Areas Covered 

1. What is Gestational Age
     – Definition, Method of Calculation, Importance
2. What is Fetal Age
     – Definition, Method of Calculation, Importance
3. What are the Similarities Between Gestational Age and Fetal Age
     – Outline of Common Features
4. What is the Difference Between Gestational Age and Fetal Age
     – Comparison of Key Differences

Key Terms 

Conception, Due Date of Pregnancy, Fetal Age, Gestational Age, Last Menstrual Period

What is Gestational Age 

Gestational age is the age of the pregnancy when calculated starting from the last menstrual period. Therefore, it is also known as menstrual age. Generally, the length of pregnancy according to the gestational age is 40 weeks. Furthermore, the regular length of pregnancy is divided into three trimesters: developmental stage, embryonic stage, and fetal stage.

Figure 1: Time Difference Between Gestational Age and Fetal Age and Organ Development

The commencement of the developmental stage of the gestational age occurs two weeks before the fetal age. It is due to the occurrence of ovulation two weeks after the last menstrual period. However, as gestational age is calculated depending on the last menstrual period, which is visible, it is a more accurate measurement in contrast to the fetal age. 

What is Fetal Age 

Fetal age is the actual age of the growing baby, starting at the time of conception. Hence, it is also known as the conceptual age. Generally, ovulation occurs two weeks after the last menstrual period. Basically, fertilization occurs close to ovulation. Therefore, the date of conception lies at the date of ovulation. Thus, fetal age is two weeks later than the gestational age. It is a less accurate measurement due to the difficulty of calculating the date. 

Similarities Between Gestational Age and Fetal Age  

• Gestational age and fetal age are two types of calculations of the age of the pregnancy.  
• Their main function is to determine the due date of pregnancy.  
• In general, they are divided into three trimesters: the developmental stage, the embryonic stage, and the fetal stage.  
• Both are important in determining the embryonic and fetal stages.  
• Moreover, they are important in determining the development of the central nervous system, heart,  ear, eyes, limbs, lip, palate, teeth, external genitals, etc. 

Difference Between Gestational Age and Fetal Age 

Definition 

Gestational age is the age of the pregnancy taken from the beginning of the woman’s last menstrual period (LMP) while fetal age is the actual age of the growing baby, starting at the time of conception.  

Also Known as 

While gestational age is also known as the menstrual age, fetal age is also known as the conceptual age. 

Time Difference 

Gestational age includes an additional two weeks time where the woman is not pregnant while fetal age is calculated after conception. 

Developmental Stage 

Moreover, the first four weeks of the gestational age belong to the developmental stage while the first two weeks of the fetal age belong to the developmental stage. 

Embryonic Stage 

The embryonic stage persists through the 5^th to 10^th weeks of the gestational age while the embryonic stage persists through 3^rd to 8^th weeks of the fetal age. 

Fetal Stage 

Furthermore, the fetal stage persists through 11^th to 40^th weeks of the gestational age while the fetal stage persists through 9^th to 38^th weeks of the fetal age. 

The Length of the Pregnancy 

The length of the pregnancy, according to the gestational age, is 40 weeks, while the length of the pregnancy according to the fetal age is 38 weeks. 

Accuracy 

Moreover, gestational age is more accurate than fetal age. 

Conclusion 

Gestational age is the age of pregnancy when calculated starting from the women’s last menstrual period. Therefore, it includes an additional two weeks of time to the actual age of the embryo. Also, the length of the pregnancy should be 40 weeks according to the gestational age. In comparison, fetal age is the age of pregnancy when calculated, starting from the conception. Hence, it is the actual age of the embryo. The length of pregnancy should be 38 weeks according to the fetal age. Therefore, the main difference between gestational age and fetal age is the starting point. 

References:

1. Curran, M. A., “Fetal Age on a Given Date Calculator.” Perinatology.com, Focus Information technology, Available Here.