There are several causes of fluid in the uterus, including endometriosis and pregnancy. Occasionally, other fertility and hormonal issues may also lead to accumulated menstrual blood and other fluids. Cysts and non-cancerous masses may also sometimes contain fluid, although this is not in the uterus directly.

One of the most common causes of fluid in the uterus is pregnancy. When a developing baby is inside the body, it is surrounded by a sac. This sac is filled with amniotic fluid and acts as a source of nutritional support and protection from the developing fetus. When the child is born, the sac usually bursts or is ruptured and the fluid escapes.

Other causes of fluid in the uterus is menstrual blood or lochia, which is a substance which occurs after pregnancy. In either of these causes, the fluid is primarily blood combined with thin tissue. With certain fertility issues, such as poly cystic ovary syndrome (PCOS) or endometriosis, sometimes this fluid can accumulate and remain in the uterus for months at a time. This may lead to hardened buildup after several months or years, and this may require a surgical procedure to remove.

Occasionally, fluid in the uterus may be caused by other hormonal conditions or growths. A very small amount of fluid may be due to normal cervical mucus in amounts larger than average. Other times fluid in the uterus may be caused by a more serious medical condition.

Cysts, which are benign growths which can occur in various areas of the body, may also sometimes contain fluid. This will likely show up as a mass rather than fluid buildup when viewed via ultrasound or X-ray. Some cysts which occur in the uterus will need to removed to ensure that they are not uterine cancer.

Excess fluid in the uterus is not considered normal and should be checked out by a gynecologist. Although most causes will turn out to be benign, it is important to rule out any serious medical conditions. This can be done in a variety of ways, including pelvic exams, ultrasound, or X-ray. In some cases, there will be no known cause of fluid, and sometimes these instances will clear up on their own over time.