Foetal measurements: how are these calculated?

Weight and length: knowing the size of the foetus


Length and weight, measuring the foetus

Periodic check-ups and ultrasound examinations: measuring the foetus enables future mothers and fathers to check that baby is growing normally, and helps them to get to know their little one.

Knowing the length and weight of the foetus: a common desire

During pregnancy, it is only natural that parents want to know if their baby is growing normally, as well as keeping track of their little one's measurements and how they may look. Periodic appointments and checks are exactly what this is all about. The weight and length of the foetus measured by ultrasound, and are two of the parameters that can indicate whether baby is developing properly.

Calculating foetal measurements

Two routine ultrasound scans are performed during pregnancy. The first, often referred to as the obstetric scan, is carried out by the end of the first trimester, and measures the cranio-sacral length, or in other words, the distance between the head and the sacrum. At this stage, it is not yet possible to measure the total length of the foetus, as he or she is curled up. From the fourteenth week onwards, the second ultrasound can be performed (the morphology scan), which allows the length of the foetus from head to heel to be measured, as well as the length of the humerus and the femur.

If you want to find out the foetal length without waiting for your next ultrasound, you can use an online calculator, entering the length of the femur as measured in the most recent ultrasound scan into the calculation system. The foetal length is approximately 7 times that of the femur. The foetal weight is calculated using the distance between the ears – or biparietal diameter – and the circumference of the abdomen. In both cases, the results are approximate.

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