In view of the large accumulation of literature concerning the upper part of the urinary tract during pregnancy, the little attention paid to the bladder under the same conditions seems remarkable. It is well known at about the fourth month of pregnancy ureteral dilatation begins, a condition of atonia that expresses itself as a relaxation of the smooth musculature of the ureters.1 This dilatation is demonstrable by pyelography and includes the renal pelves. It is probably caused by the increased elaboration of progesterone at the beginning of the second trimester as the placenta takes over the functions of the corpus luteum.
At this time the bladder undergoes changes in keeping with those in the upper part of the urinary tract. With the hyperemia affecting all the pelvic organs, plus a definite hyperplasia of muscular and connective tissue elements, there is an elevation of the trigone and a thickening of
Seidner HM, Arnkoff M, Mills GY. URINARY RETENTION IN PREGNANCY. JAMA. 1952;149(5):425-426. doi:10.1001/jama.1952.02930220015006