Fetus length

Knowing the length of the fetus during the various stages of pregnancy makes it possible to assess its gestational age, growth and health status, highlighting any morphological anomalies.

Obstetrical ultrasound

Obstetric ultrasound, performed by the end of the first trimester of pregnancy, has the task of investigating the number of fetuses, their vitality and the gestational age. In this first phase, in fact, the cranio-caudal length (head - sacred) appears almost constant in fetuses of identical dating.

Therefore, by detecting this parameter using ultrasound images, the doctor can establish the gestational age and the expected date of birth, all with an excellent degree of approximation. If measured between the 7th and 11th week, for example, the cranio-caudal length (CRL) has an accuracy of more or less 3/4 days in 95% of cases.

The gestational age of ultrasound thus calculated may vary slightly from that established on the basis of the last menstruation (amnestetic period).

For the same purpose and in the same way, the biparietal diameter (BPD), which when measured between the 12th and 16th week, offers an accuracy similar to the cranio-caudal length (plus or minus 3 / 4 days in 95% of cases).

Morphological ultrasound

After the first trimester of pregnancy, the chances of recognizing fetal malformations increase significantly. Therefore, around the twentieth week of pregnancy, a second ultrasound is performed, called morphology, which also has the task of evaluating a whole series of biometric parameters. For example, the reduction in the length of the humerus or femur compared to the reference values is frequently observed in trisomy 21 (Down syndrome). The measurement of BPD, femoral length, head circumference and abdominal circumference (CA) is also important for establishing or confirming the gestational age of the infant.

Biometric ultrasound

In the third trimester of pregnancy (between the 28th and 32nd week) the so-called obstetric or biometric ultrasound is performed, which investigates fetal growth and anatomy again, allowing to detect any malformations that escaped the previous morphological ultrasound. This ultrasound scan is never used to date pregnancy, a parameter that must already have been clearly established in previous ultrasound investigations. Biometric ultrasound, rather, evaluates the adequacy of fetal growth based on standard values, referring to the gestational age estimated in the previous controls; among the parameters detected, we mention the aforementioned BPD (biparietal diameter), femur length (LF), cranial circumference (CC) and abdominal circumference (CA).

Gestational age calculation

Below are some calculation modules to establish the gestational age in relation to specific fetal biometric parameters. The results should not replace the opinion and advice of health personnel responsible for evaluating the health of the fetus.

BIPARIETAL DIAMETER (BPD)