The normal range of SD value at 31 weeks of pregnancy should be <3. SD value, also known as umbilical artery flow value, is a frequent indicator in ultrasound results, which refers to the ratio of peak end-systolic (S) to peak end-diastolic (D) of umbilical artery in late pregnancy, and is related to fetal blood supply, and is an important indicator to assess whether the fetus is developing normally. If the SD value is high, it will affect the fetal development and also reflect different degrees of pathological problems of the mother and fetus. For the mother, it may indicate the presence of gestational diabetes mellitus, gestational hypertension and other diseases, which may be detrimental to the subsequent pregnancy and delivery. If you have SD abnormalities, you should seek timely medical treatment, oxygen therapy if necessary, regular review, and pay attention to your health care in general.