Development of the levator ani muscle in human fetuses.

The development of the levator ani muscle was studied in 300- to 700-microns thick sections through the pelves of 9- to 37-week-old fetuses and newborn children. During early fetal development, the anlage of the levator ani muscle can already be subdivided into three portions. The planes of cleavage between them become clearly evident in fetuses during the middle trimester when all points of origin and insertion of the levator ani muscle can be recognized. The tendinous arch of the pelvic fascia plays an important role in the differentiation between the pubococcygeus and the iliococcygeus parts of the levator ani muscle. The funnel-shaped form of the pelvic diaphragm is completed by about the 14th week of fetal development. Differences between the male and female levator ani muscles are already marked before birth.