Gestational Age Vs Foetal Age

Monitoring the growth of the foetus is an important task for obstetricians worldwide. Therefore the obstetric community has developed a set of measures and values to asses and compare growth. Gestational age and foetal age are two important values which have a great bearing on pregnancy decision making.

Gestational age is defined as the age of the pregnancy calculated from the last menstrual period. Hence this period also includes the time from the last menstrual period to the day of ovulation/fertilisation. This period could vary between 11-20 days in women with regular 28 day cycles.

In women who have cycles which are longer or shorter than the normal 28 day cycles, this period can increase or decrease from the above values. Gestational age is difficult to calculate in women who have irregular cycles.

Gestational age forms the baseline from which all observations regarding foetal growth and development are compared. It is also the basis for the calculation of the expected date of delivery. The EDD is calculated from the under mentioned formula Age calculator .

EDD=LMP+ Nine months and seven days.

The calculated EDD is used throughout pregnancy to guide decisions, especially those regarding termination of pregnancy due to foetal or maternal causes.

On the other hand foetal age is the actual age of the foetus. Therefore this value will be lower than the gestational age by 11-20 days. In obstetrics all references to pregnancy are usually in gestational age rather than fetal age, except in cases where LMP is not known.

This situation is commonly seen when women get pregnant immediately after delivering a baby. After delivery due to the action of the prolactin produced by breast feeding, ovulation remains suppressed for a period of 3-6 months. Therefore women usually have amenorrhea for a period of 3-6 months following delivery of a baby. However onset of ovulation is very unpredictable and this may lead to many women getting pregnant during this period.

Since there is no LMP to calculate gestational age, one has to depend on the foetal age for pregnancy decision making. Foetal age is estimated by ultrasound. During the scan measurements of the head circumference, crown rump height, femur length, and abdominal circumference can be used make an assessment of the foetal age. The accuracy of ultrasound in assessing foetal age increases as the pregnancy progresses.

Therefore to summarise, gestational age is calculated from the date of LMP and is the standard reference in obstetrics. Foetal age is the actual age of the foetus and this is lower than the gestational age by 11-20 days. The foetal age is a useful replacement when the gestational age is difficult to calculate.

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