4Dfl is an abbreviation for femur length. Many of the fetal limb bones can be visualized by ultrasound and used to estimate gestational age. The femur is the longest, least mobile and most easily visualized of these bones and is therefore the most widely used, while other bones such as the humerus, tibia and radius can also be detected. The best time to measure the femur is when the fetal head is not suitable for gestational age estimation, usually starting after 15 weeks of gestation. The length of the femur is measured by first finding the rump, then turning the probe to show the full length of the femur, and finding the longest axis of the femur, which is the distance from the distal epiphysis to the apex of the proximal bevel. The femur is used to estimate the gestational week with an accuracy similar to that of the biparietal diameter and to estimate the time of stillbirth, but it cannot be used to estimate gestational age in cases of femoral dysplasia, fractures, etc.