This article examines possible factors that would explain the frequency of extrauterine pregnancies in women and their rarity in domestic mammals. About 1% of pregnancies are extrauterine, and of these about 98% are tubal, a condition almost unknown among other mammals. Ectopic pregnancy increases in women with age and parity, but among domestic mammals the rotation of females used for reproduction is usually rapid. Women undergoing surgical interventions for tubal pathology due to congenital conditions, infections, or other causes have a high rate of ectopic pregnancy, but mammals with tubal pathology generally do not reproduce. Ectopic pregnancies are not seen in domestic animals subjected to experimental tubal surgery. In women but not in mammals the administration of gonadotropins to promote ovulation is associated with a higher rate of ectopic pregnancy. The failure of some types of contraceptives, such as tubal sterilization, is associated with ectopic pregnancy in women but no comparable situation is found in domestic animals. It is likely that the mechanisms of reproduction in mammals, unlike those in women, inhibit the development of ectopic pregnancies. Several factors have been identified which distinguish women and domestic mammals: in women the presence of the embryo is not required in the uterus for luteal progesterone secretion to take place, the embryo is capable of surviving in either the tubes or the uterus, the placenta is hemochorial, and implantation is interstitial. The 4 features have not been found together in any other mammal, and it appears that the more numerous the differences in any mammal, the smaller the chances that a tubal pregnancy will occur. 2 approaches to study of the differences in humans and mammals are recommended: use of an experimental model more similar to the human, and use of the usual experimental species to analyze the relative importance of each of the factors identified.