What is the relationship between the lower segment of the uterus and the isthmus of the uterus?

Under normal circumstances, the lower uterine segment develops from the uterine isthmus during pregnancy. The detailed explanation is as follows:

In the non-pregnant state, the uterine isthmus is a relatively narrow area between the body of the uterus and the cervix, measuring approximately 1 cm in length. After conception, the uterine isthmus gradually elongates, stretches, and becomes thinner. By late pregnancy, especially during labor, the uterine isthmus can extend to 7–10 cm, forming the lower uterine segment. This transformation occurs due to hormonal changes during pregnancy, which cause hypertrophy and proliferation of uterine smooth muscle cells and remodeling of the tissue structure of the uterine isthmus. Additionally, as the fetus grows and develops, the intrauterine pressure gradually increases, further promoting the elongation of the uterine isthmus to form the lower uterine segment. The lower uterine segment plays an important role during childbirth, adapting to the dilation demands as the fetus passes through the birth canal.

Throughout pregnancy, prenatal examinations must be conducted strictly according to the physician's instructions, with close monitoring of changes in the thickness and morphology of the lower uterine segment. Particularly for pregnant women with a history of cesarean section or other uterine surgeries, regular prenatal checkups are essential for timely detection of any abnormalities in the lower uterine segment, such as thinning or tenderness, allowing medical professionals to implement appropriate measures to ensure the safety of both mother and child.