Fetus length calculation (and fetus length table)

Fetus length calculation

There is a linear relationship between fetal femur length on ultrasound and head-to-heel length. The formula for calculating fetal length in centimeters is 6.18 + 0.59 x femur length in millimeters.

Fetus length calculator

Take an ultrasound of the fetus and fill out the form below. Fetal length (FL) is sometimes referred to on ultrasound scans as F or as FE.

Enter the length of the femur (FL): mm

Fetal Length Estimation: cm

Ultrasounds and measurements of the fetus

Finally, you should know that the measurement of a fetus is usually carried out:

  1. at the end of the first trimester, during the first obstetric ultrasound which allows the fetus contained in the uterus to be viewed on a monitor;
  2. around the 20th week of pregnancy using morphological ultrasound, which also allows you to examine some parts of the fetal body;
  3. around the 28th – 32nd week of pregnancy, through a second obstetric ultrasound or a biometric ultrasound (this is aimed at evaluating the overall fetal growth).

During the first obstetric ultrasound, the healthcare professional measures the cranio-caudal length (CRL), i.e. the length of the baby from the head to the sacrum.

While during the morphological and biometric ultrasound its “complete” length is measured, i.e. from the head to the heel.

The fetal femur length (FL) is used starting with second trimester ultrasound.

Fetus length table

Based on a 2019 scientific study (Hill, 2019), we have prepared a table useful for obtaining the length and average weight of the fetus from the eighth week until delivery.

Keep in mind that boys tend to be longer and heavier than girls, and that every baby is different.

So don’t worry if ;yours; measures are different from those listed below: in case of doubts or concerns always refer to your gynecologist, obstetrician, or family doctor.

In fact, all the information you find online and also on our site can be complementary to that of healthcare personnel and can never replace them.

Pregnancy week Length (cm) Mass (g) Note
8 week 1,6 cm 1 g CRL
9 week 2,3 cm 2g CRL
10 week 3,1 cm 4g CRL
11 week 4,1 cm 7g CRL
12 week 5,4 cm 14 g CRL
13 week 7,4 cm 23g CRL
14 week 8,7 cm 43 g CRL
15 week 10,1 cm 70 g CRL
16 week 11,6 cm 100 grams CRL
17 week 13 cm 140 g CRL
18 week 14,2 cm 190 g CRL
19 week 15,3 cm 240 g CRL
20 week 25,6 cm 300 grams head to heel
21 week 26,7 cm 360 g head to heel
22 week 27,8 cm 430g head to heel
23 week 28,9 cm 500 g head to heel
24 week 30 cm 600 g head to heel
25 week 34,6 cm 660 g head to heel
26 week 35,6 cm 760 g head to heel
27 week 36,6 cm 875 g head to heel
28 week 37,6 cm 1 kg head to heel
29 week 38,6 cm 1.2kg head to heel
30 week 39,9 cm 1,3 kg head to heel
31 week 41,1 cm 1,5 kg head to heel
32 week 42,4 cm 1,7 kg head to heel
33 week 43,7 cm 1,9 kg head to heel
34 week 45 cm 2,1 kg head to heel
35 week 46,2 cm 2.4kg head to heel
36 weeks 47.4cm 2.6kg head to heel
37 week 48.6cm 2.9kg head to heel
38 week 49.8cm 3.1kg head to heel
39 week 50.7cm 3.3kg head to heel
40 week 51.2cm 3.5kg head to heel
41 week 51.5cm 3.6kg head to heel
42 week 51.7cm 3.7 kg head to heel

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform, Maternicity.com.

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