For a certain length of time the idea existed that during labor the entire pelvic ring is incapable of any change in size. Clinical observations, however, support the assumption that during pregnancy and labor not only is a certain amount of movement possible in the pelvic joints, but a distinct stretching or widening takes place. Some of these observations are directly confirmable; others can be made during actual labor.
The normal articular cavities of the symphysis pubis and the sacro-iliac joints are the anatomic basis for the movements of the pelvic joints in pregnancy and labor. During pregnancy a relaxation of the strong ligaments in the sacro-iliac joints and the symphysis pubis occurs and makes possible an increased mobility. The importance of this increased mobility lies in the possibility of a change in size of the pelvic ring. This relaxation also prevents, to a certain degree, the forcible rupture of
LANG FJ, HASLHOFER L. CHANGES IN THE SYMPHYSIS PUBIS AND SACRO-ILIAC ARTICULATIONS AS A RESULT OF PREGNANCY AND CHILDBIRTH. Arch Surg. 1932;25(5):870-879. doi:10.1001/archsurg.1932.01160230053004