I do not believe that it is possible for an ectopic pregnancy to develop primarily in the peritonealcavity, for the ovum can not be lodged securely in any one place sufficiently long to form chorionic attachments, and the peritoneal secretion would very soon destroy its vitality; the one or more cases of reported abdominal pregnancy, after removal of the uterus, were primarily tubal or ovarian, these organs not having been removed with the uterus.
Ectopic pregnancy must be primarily tubal, tubo-ovarian, or ovarian, but as it can not develop to term in any of these structures, it ruptures either into the folds of the broad ligament or into the peritoneal cavity; and if the chorionic attachments are not then destroyed, there is no reason why the pregnancy may not continue to term.
I believe that most cases reported as intra-peritoneal pregnancy are extra-peritoneal, having developed in the folds of the
WATHEN WH. ECTOPIC PREGNANCY, WITH REPORT OF A CASE OF OVARIAN PREGNANCY.. JAMA. 1901;XXXVII(9):548–550. doi:10.1001/jama.1901.62470350004002a