AC – Abdominal Circumference APTD – the thickness between the anterior and posterior abdomen, also known as the anterior-posterior abdominal diameter. This measurement is needed to check the development of the fetal abdomen and to determine the weight of the fetus. BDP – the length of the longest part of the head between the left and right sides of the biparietal diameter, also known as the “large transverse head diameter”. It is often used to predict the due date when the CRL is not available in the early stages of labor, and is often needed to determine the weight of the fetus after the midterm stage. CRL – head-rump length is the head-rump distance, which indicates the largest long axis measured parallel to the longitudinal axis of the fetal body, and is mainly used to determine the gestational age from 7 to 12 weeks of gestation. HC – Head circumference FL – Femoral length The length of the fetal thigh bone, also known as the “thigh bone length”. The thigh bone is the length of the thigh from the root of the thigh to the knee. The FL is usually measured at about 20 weeks of gestation to check the development of the fetus. GS – fetal sac In women with regular menstruation, the gestational sac can be seen in the uterine cavity by ultrasound at 35 days of menopause. The gestational sac is about 2 cm in diameter at 1.5 months of pregnancy and about 5 cm at 2.5 months. It is normal for the location of the sac to be in the fundus, anterior wall, posterior wall, upper part and middle part of the uterus; round, oval and clear shape is normal; if the sac is irregularly shaped, fuzzy and located in the lower part, the pregnant woman may have to abort if she has abdominal pain or vaginal bleeding at the same time. HL – humeral length TTD – the width of the abdomen, also known as the “transverse abdominal diameter”. It is taken after 20 weeks of gestation, along with the APTD, to check the development of the fetus. Sometimes the area of the abdomen is also measured GP – Placenta grading, generally the placenta is graded into 0, I, II, III and sometimes III AFI – Amniotic fluid index, during the ultrasound, the amniotic fluid is divided into 4 areas: upper, lower, left and right, with the umbilicus of the pregnant woman as the center. The normal value of amniotic fluid index in late pregnancy is 8~18(24) cm. S/D – the ratio of fetal umbilical artery systolic pressure to diastolic pressure, related to fetal blood supply, this ratio will be abnormal when the placenta is not functioning well or the umbilical cord is abnormal, in normal pregnancy, with the increase of fetal needs with the increase of gestational weeks S decreases, D increases, so that the ratio decreases, near full-term pregnancy S/D is less than 3.