Human fetal anatomy of the coccygeal attachments of the levator ani muscle.

In contrast to the attachments to the pubis and rectum, there is little information on fetal development of the coccygeal attachment of the levator ani muscles. We find that at 9 weeks, the coccygeus muscle is a large muscle facing the piriformis or gluteus maximus and inserting onto the ischial spine, whereas the levator ani is restricted to the area near the pubis. By 12 weeks, the levator ani also obtains attachment to the ischial spine immediately ventral to the coccygeus muscle. The most superior part of the coccygeus muscle occupies a space at an angle between the pelvic splanchnic and pudendal nerves. Notably, medial to the coccygeus muscle, a third parasagittal muscle (previously termed the sacrococcygeus anterior) appears by 12 weeks, increases in mass by 18 weeks, and connects and mixes with the dorsal end of the levator ani by 18-20 weeks. Thus, the coccygeal attachment of the levator ani appears not to depend on the dorsal extension of the muscle itself but on fusion with the sacrococcygeus anterior. Therefore, the final levator sheet is formed medial (internal) to the coccygeus muscle and originates from two distinct anlage.