The lower limit of normal at 39 weeks gestation for femur length of 6.6cm needs to be fully assessed by a doctor. Femur length (FL) is the length of the fetal thigh bone, and fetal growth can be assessed during pregnancy by ultrasound monitoring of FL and indicators such as biparietal diameter and abdominal circumference. FL at 39 weeks of pregnancy is 6.5-7.7cm, so when the ultrasound suggests that FL is 6.6cm is the lower limit of normal, or within the normal range, which is equivalent to 7.4% of the average value, which does not indicate that the fetal development is abnormal, and should be assessed in conjunction with the overall development of the fetus and the height of the pregnant woman and other genetic factors. When you are 39 weeks pregnant, you should follow the doctor’s instructions to take obstetric checkups on time, count the fetal movements to self-evaluate the intrauterine situation of the fetus, and also take moderate physical exercise under the doctor’s guidance to facilitate natural delivery, such as walking, sitting on a birthing ball, and so on. When irregular abdominal pain, redness, vaginal fluid, etc. occurs, you should go to the hospital in time, and get ready for delivery.