Changes in the physical properties of the uterine cervix of the rat during pregnancy

During pregnancy changes take place in the walls of the birth canal, making it more distensible and so facilitating the passage of the foetus at parturition. Investigators of this subject have concentrated mainly on the pelvic bones and interosseous ligaments, in particular the symphysis pubis (see Hisaw & Zarrow, 1950; Frieden & Hisaw, 1953); little work has been done on the uterine cervix which is the narrowest part of the birth canal. We have examined changes in the physical properties of the uterine cervix of the rat during pregnancy. The results indicate that these mainly affect the connective tissue framework and consist, first, of an increase in size so that the diameter to which it can be expanded by relatively low forces is increased, and secondly, of a change in the nature of the material which resists mechanical stretching. This material in the non-pregnant animal is only slightly distensible even by strong forces acting for long times. At the end of pregnancy, however, it shows properties of a very viscous material and distends slowly and progressively even under low forces. The change in the physical properties of the cervix does not begin until the 11-12th day of pregnancy and then is progressive; it involves a relative diminution in the amount of smooth muscle present and a fall in the concentration of collagen. Preliminary accounts of some of this work have already been published (Harkness & Harkness, 1956; Harkness, 1957).