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What is the difference between gestational age and fetal age?


Gestational Age vs Fetal Age



The two terms, gestational age and fetal age, are used to describe the age of the growing fetus after the egg has been fertilized by a sperm. They are calculated differently. The gestational age is calculated from the first day of the last menstrual period. This calculation is used to detect the expected due date of the pregnancy. The fetal age is calculated from the actual date the conception takes place. Ideally, conception takes place approximately two weeks later the last menstrual period and so the fetal age should always be two weeks lesser than the gestational age.

Significance of the gestational age

         The gestational age is important as it helps to calculate and expect the development of the fetus. The total length of the gestational age is divided into three trimesters or roughly 3 months each. The first trimester lasts from first week to the end of thirteenth week; the second trimester extends from the fourteenth to the end of twenty sixth weeks. The last trimester lasts up to the thirty eighth or the forty second week or till delivery takes place. The fetal development varies in different trimesters and hence the gestational age and the fetal age should mostly correlate with the fundal height and the growth of the fetus. The gestational age should match the height of the fundus of the uterus as the body prepares for pregnancy and this can examined per abdomen and confirmed. If the uterine fundal height is lesser than the gestational age it suggests the doctor to look out for any abnormality. The fetal weight if less than the 10th percentile at a gestational age proves that the fetus is small for gestational age. A fetus born before the 37th week of gestation is called as preterm baby whereas if the fetus is born after the 40th week, it is called as post term baby.


Significance of the fetal age

           The fetal age is important for the doctor to know if the fetus is growing at a normal rate and if the fetal internal organs are growing normally, without any anomalies. The fetal growth should always match up the fetal age and can be measured once a month after pregnancy begins. An ultrasound helps to find the fetal age and hence, ultrasounds should be done at least once each month after the first trimester. If some complications occur the fetal age will allow us to work up on early treatment which can be done to prevent fatal complications for the fetus. Certain medical tests like the nuchal rigidity tests and anomaly scan have to be done around the 12th and the 13th week to rule out any abnormal growth of the fetus. Thus, assessing fetal age is essential to ensure appropriate fetal growth.

Gestational age can be easy to calculate as one can find out the first day of the last menstrual period but fetal age is based on suppositions as it uses the date when actual fertilization takes place for calculations. There are no external visible signs that fertilization has taken place inside the womb in the first week or two. Gestational age can be calculated manually and by examining the uterine fundal height but fetal age can be calculated by ultra sonography alone. Both the ages should match, is the main aim of the doctors.



Gestational age calculates the period of gestation from the first day of the last menstrual period and is used to calculate the due date of delivery chiefly. Fetal age calculates the exact age of the fetus based on the calculation using the date the fertilization actually took place.

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1 Comment

  1. So I didn’t know my lmp and I went to Capps pregnancy clinic. They said my baby measured at 20 weeks and 2 days. Is that the fetal age? Or does the ultrasound automatically add 2 weeks? Why would it add two weeks if that’s the. Awe if they are determining the age of the unborn baby? Please help!

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