The normal range of SD value at 31 weeks of pregnancy should be <3. SD value, also known as umbilical artery blood flow value, is an indicator that often appears in the ultrasound results, which refers to the ratio of the peak systolic value (S) to the peak diastolic value (D) of umbilical artery in the late stage of pregnancy, which is related to the blood supply of the fetus and is also an important indicator for assessing whether the fetus is developing normally or not. Generally with the increase of pregnancy, the SD value should gradually become lower, under normal circumstances, every increase of 1 week of pregnancy, SD value will be reduced by 0.1. If the value is high, it will affect the development of the fetus, and it will also reflect that the mother and the fetus have different degrees of pathological problems. when the SD value is high, for the fetus, it can appear that there is a poor flow of blood in the umbilical artery, which can impede the transmission of nutrients in the umbilical cord, which is not conducive to the growth and development of the fetus and may even cause intrauterine hypoxia; for the mother, it may indicate the presence of gestational diabetes mellitus, gestational hypertension and other diseases, which is unfavorable for subsequent pregnancy and delivery. If there is any abnormal SD value, you should pay attention to timely medical treatment, oxygen therapy if necessary, regular review, and pay attention to health care in general.