What is the boundary between the pelvic cavity and the abdominal cavity?

The boundary between the abdominal and pelvic cavities is essentially defined by the upper margin of the pelvis. In a broad sense, the abdominal cavity includes the pelvic cavity. The division between the abdomen and pelvis is generally taken at the pelvic inlet or, alternatively, at the base of a distended bladder, as detailed below:

The pelvic and abdominal cavities are not separated but are interconnected. The pelvic cavity typically refers to the space within the bony pelvis and contains organs such as the uterus, adnexa, and parts of the intestine. The abdominal cavity houses organs including the liver, pancreas, intestines, kidneys, and stomach. In fact, the abdominal cavity encompasses the pelvic cavity, which is why the pelvic region is also referred to as the lower abdomen. However, because the pelvic cavity is surrounded by the bony pelvis, it is often designated separately as the "pelvic cavity." Localized pelvic pain is commonly attributed to inflammation of the uterus or its adnexa.

The abdominal and pelvic cavities are continuous with each other, whereas the abdominal and thoracic cavities are not directly connected; they are separated by the diaphragm. The true pelvic cavity is anatomically bounded by the superior edge of the pubic symphysis, the iliopectineal line, and the superior margin of the sacral promontory, dividing the pelvis into two parts: the false pelvis (greater pelvis) and the true pelvis (lesser pelvis). The false pelvis, located above the pelvic brim, is considered part of the abdominal cavity. Therefore, in this context, the abdominal and pelvic cavities are continuous and effectively form one integrated space.