A 16-year-old female is brought to the emergency department with severe abdominal pain and fever. Laboratory examination is remarkable for an elevated white blood cell count and a positive test for pregnancy. Colpocentesis is performed to ascertain the presence of blood in the pelvis from the ruptured ectopic pregnancy. Through which of the following structures does the needle need to be inserted?

It is very likely that the ectopic pregnancy ruptured into the rectouterine pouch, also known as the pouch of Douglas. The most direct route to the rectouterine pouch is through the vaginal wall at the posterior vaginal fornix. It is unlikely that the pregnancy would have occurred in the vesicouterine space because the transfer of ova from the ovary to the fimbriae occurs on the posterior side of the broad ligament. Therefore, it would not be advisable to attempt initially to insert a needle into the vesicouterine space. Inserting a needle through the anterior fornix into the endocervical canal would lead one into the uterine cavity, with the probability of other undesirable consequences. The urogenital diaphragm is a closed space in the perineum. Entering a vestibular gland with a needle would not be near the location of ectopic pregnancy.