Ectopic pregnancy

In approximately 1% of all recognised pregnancies, the conceptus implants in a site other than the uterine cavity. The vast majority (95–97%) of such ectopic gestations occur in the fallopian tube. Less common sites are the ovary, cervix and peritoneal cavity and occasional cases of implantation occur in the vagina, liver or spleen.

Tubal pregnancies are predisposed to by any factor which impairs the ability of the tube to transport the fertilised ovum. Hence, congenital abnormalities of the tube, failed tubal sterilisation, the use of a progesterone-only contraceptive pill, salpingitis isthmica nodosa, reconstructive tubal surgery and, most importantly, post-inflammatory tubal damage, are all associated with an increased incidence of tubal pregnancy. In about 50% of such pregnancies the tube is, however, histologically normal. It has been argued that, in such cases, conception occurred during a cycle in which there was delayed ovulation and a short, inadequate luteal phase. Consequently, when the fertilised ovum reached the uterine cavity it had not yet developed to a stage when it was secreting enough human chorionic gonadotrophin (hCG) to prevent decay of the corpus luteum and was flushed back into the tube by a reflux of menstrual blood subsequent to menstrual bleeding. This hypothesis is supported by the fact that tubal gestation occurs only in species which menstruate and by the not uncommon finding of the corpus luteum of pregnancy being on the opposite side to that of a tube containing a pregnancy.