The 4D ultrasound biparietal diameter minus femur length equals 1cm and is normal depending on the length of the pregnancy. If it is 12 weeks it is normal, above 12 weeks it may not be normal. Biparietal diameter refers to the distance between the widest parietal bones of the fetus, which is the maximum transverse diameter of the fetal head. Femur length refers to the length of the fetus’s thigh bone, and is also an indicator of the size of the fetus. The height of the fetus is determined by the length of the femur, so the biparietal diameter and femur length are used to measure the growth and development of the fetus. Biparietal diameter minus femur length refers to the biparietal diameter of the fetus minus the length of the fetal femur, if it is at 12 weeks, the biparietal diameter of the fetus is 1.7 cm, and the femur length is 0.7 cm, so at this time, the 4D ultrasound biparietal diameter minus the femur length equals to 1cm is normal. You need to take a timely pregnancy test during pregnancy, and any abnormalities in the test results are recommended to seek timely medical attention to clarify the cause.