If the ultrasound during pregnancy suggests a fluid dark area in the uterus, it usually suggests that there is blood accumulation in the uterine cavity, which may be a normal physiological phenomenon of the fertilization of the egg, or it may be an omen of preterm miscarriage. 1. Normal physiological phenomenon: after the sperm and egg combine to form a fertilized egg, it usually enters the uterine cavity from the fallopian tube to settle and develop, and the endometrial lining of the uterus may be slightly damaged or bleed when the fertilized egg settles, and the ultrasound may indicate a fluid dark area in the uterine cavity, which is generally a normal physiological phenomenon, and it is recommended that regular rechecks of the blood HCG, progesterone and ultrasound should be conducted to ascertain the status of the development of the embryo. 2. Pre-eclampsia: fluid dark area in the uterine cavity at the beginning of pregnancy may be caused by progressive bleeding in the uterine cavity, which is often accompanied by abdominal pain, vaginal bleeding and other discomforts of pregnant women, in this case, there may be the risk of pre-eclampsia, we should further check the blood HCG, estradiol and progesterone values, and improve the thyroid function test, in order to find out the causes of pre-eclampsia and, according to the results of the examination, to give birth control treatment, and Pay attention to regular ultrasound review.