Which structure does NOT form pa of the boundary defining the trigone of the bladder?

The trigone of the bladder is bounded by the openings of the left and right ureteric orifices superolaterally, the interureteric crest between the openings of the ureters and the internal urethral orifice inferiorly. The urachus is the structure that joins the apex of the fetal bladder to the umbilicus. After bih, it becomes the median umbilical ligament.