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.
Ultrasounds and measurements of the fetus
Finally, you should know that the measurement of a fetus is usually carried out:
- 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;
- around the 20th week of pregnancy using morphological ultrasound, which also allows you to examine some parts of the fetal body;
- 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.