A small amount of fluid in the uterine cavity in early pregnancy may be caused by physiological phenomena, ectopic pregnancy, endometritis and other reasons. 1. Physiological phenomenon: the small amount of plasma secreted by the peritoneum that protects the surface of the organs and reduces the friction between the organs can also flow into the pelvis and lead to the formation of effusion, which belongs to the physiological phenomenon and doesn’t need special treatment. 2. Ectopic pregnancy: it is mainly caused by inflammatory damage to the fallopian tube as well as abnormal function and contraceptive failure. The patient may have a small amount of uterine metamorphosis, which may result in a small amount of fluid in the uterine cavity. 3. Endometritis: refers to the bacteria break through the defense of the cervix and invade the endometrium and inflammation occurs. Inflammatory cells tend to gather in the uterine cavity, resulting in increased secretion of various inflammatory factors, increased vascular permeability of the endometrial wall, the accumulation of inflammatory exudate in the uterine cavity, so that a small amount of intrauterine fluid can occur. In addition, it is recommended that pregnant women with a small amount of fluid in the uterine cavity in early pregnancy go to the hospital in time and follow the doctor’s instructions for treatment to avoid delaying the condition.