Low HCG on the 8th day of pregnancy may be caused by preeclampsia, embryonic dysplasia, ectopic pregnancy and so on. 1. Pre-eclampsia: The trophoblast cells of the fertilized egg begin to secrete HCG about 1 day after the egg is laid. When pre-eclampsia occurs, there will be abdominal pain, vaginal bleeding and other clinical manifestations, and the blood HCG test value will be low or not rise, so it is necessary to follow the doctor’s advice to give active treatment to preserve the fetus. 2. Embryonic dysplasia: when ultrasound examination indicates that the embryo is dysplastic, the blood HCG value will be low or will be doubled. 3. Ectopic pregnancy: when the pregnancy is ectopic, the blood HCG will be maintained at a low level, and there is no doubling of the performance of the test in the interval of 2-3 days, and it can be combined with the ultrasonography to make a clear diagnosis. When HCG is low on the 8th day of pregnancy, it needs to be treated promptly and actively under the guidance of a doctor.