Low protein S activity measurement suggests that the patient’s coagulation function is poor, usually due to genetically inherited reduced protein S activity, but also may be related to the severe reaction to pregnancy in pregnant women. Protein S is an important protein in the body involved in the process of blood coagulation, and protein S activity is associated with normal blood coagulation. When protein S activity measurements are lowered, blood clotting is impaired, and patients with severe disease may show signs of difficulty in blood clotting or spontaneous bleeding (e.g., spontaneous skin bruising, etc.). Patients with congenital hereditary factors that lead to decreased protein S activity need to go to the hematology department in time for diagnosis and treatment after completing relevant examinations. In pregnant women, low protein S activity may occur after severe vomiting, which is temporary and will gradually return to normal after the vomiting response improves. Low protein S activity may also be caused by liver disease, and prompt medical consultation is recommended.