The interstitial (cornual) portion of the fallopian tube is actually within the muscular wall of the uterus. This tissue has an abundant blood supply which may allow an ectopic pregnancy (in the cornual region of the tube and uterus) to become quite large.
Surgical removal of a cornual ectopic pregnancy can result in massive bleeding. A partial (hemi) hysterectomy may be required to remove these pregnancies due to their location and size. Great care must be exercised to devascularize the surgical site during removal. A trial of medical management (such as methotrexate) is often considered for these pregnancies.
Implantation into the interstitial portion of the fallopian tube is (thankfully) uncommon.