From the Research
The upper and lower uterine segments have distinct anatomical features that change throughout pregnancy, with the upper segment characterized by thick muscular walls and the lower segment featuring thinner walls with less elastic tissue, as described in the most recent study on uterine development 1. The upper uterine segment, comprising most of the uterine body, is characterized by thick muscular walls with interlacing myometrial fibers arranged in three layers: external longitudinal, middle oblique, and internal circular. This segment contains abundant elastic tissue and blood vessels, allowing for significant expansion during pregnancy while maintaining contractile strength during labor. The endometrium in this region is thicker and undergoes cyclic changes during menstruation. In contrast, the lower uterine segment consists of the isthmus and upper cervix, featuring thinner walls with predominantly longitudinal muscle fibers and less elastic tissue. This segment has a narrower lumen, less vascular supply, and thinner endometrium. During pregnancy, the lower segment gradually develops and thins, particularly in the third trimester, forming a passive segment that dilates during labor rather than actively contracting like the upper segment. This anatomical distinction is clinically significant for cesarean section incisions, with lower segment incisions being preferred due to reduced bleeding and lower risk of rupture in subsequent pregnancies, as supported by studies on cervical cerclage and uterine biomechanics 2, 3, 4. Key features of the upper and lower uterine segments include:
- Upper segment: thick muscular walls, abundant elastic tissue, and thicker endometrium
- Lower segment: thinner walls, less elastic tissue, and thinner endometrium The development of the human uterus, including the formation of the upper and lower segments, is a complex process that occurs during fetal life and continues to change throughout pregnancy and after menarche, as discussed in a recent review on uterine development 1.