Under normal circumstances, the ovum is fertilized by the sperm in the ampullary portion of the uterine tube, whence it passes into the uterine cavity prior to implantation.
The ovum divides rapidly, differentiating into (1) the embryo, which develops over the next 40 weeks into the full-term fetus; and (2) the trophoblast, which invades the progestational endometrium to form the placenta. The developing fetus becomes enclosed in an amniotic cavity that contains amniotic fluid. The amniotic cavity is lined by amnion and chorion (fetal membranes). The amnion also covers the fetal surface of the placenta and umbilical cord. The chorion is accentuated at one pole of the uterus to form the placenta, which is usually located in the fundus of the uterus. The fetus is connected to the placenta by the umbilical cord.
An ectopic pregnancy is one in which implantation of the fertilized ovum occurs at a site other than the uterine cavity (Figure 55-1).
Sites in which ectopic pregnancies may occur.
Tubal pregnancies are common, representing about 0.5% of all pregnancies. The incidence is believed to be increasing, possibly as a consequence of the general increase in the incidence of pelvic inflammatory disease, which causes tubal narrowing and adhesions that interfere with passage of the fertilized ovum. Endometriosis involving the uterine tube is also associated with an increased risk of tubal pregnancy. In many cases of tubal pregnancy, however, no etiologic factor is identified.
In spite of the abnormal implantation site, the ovum develops normally in the first few weeks, forming a placenta, an embryo, and an amniotic sac. Later development is greatly impeded by the lack of space, poor vascular supply, and limited placental size.
Rupture of the tube containing the pregnancy frequently occurs 2–6 weeks after fertilization, causing massive, potentially fatal intraperitoneal hemorrhage (Figure 55-2). This may occur so early in the course of the pregnancy that the patient may not have missed a menstrual period. The embryo may still be alive (in which case the pregnancy test for serum or urinary human chorionic gonadotropin (hCG) will still be positive). In most cases, the released embryo dies soon after tubal rupture. When rupture of a tubal pregnancy releases a live embryo into the peritoneal cavity, it can move to a secondary implantation site on the peritoneal surface (secondary abdominal pregnancy). Very rarely, such a pregnancy progresses to term, although of course normal delivery cannot occur.
Tubal pregnancy, showing hemorrhagic mass in the ampulla of the uterine tube. No products of conception are seen in this photograph, but they were identified on microscopic examination.