Pregnancy- Some Unanswered Questions

The definition of pregnancy as the state of a female in the time interval between fertilization of her ova and the expulsion of the foetuses at term, lays focus on the maternal organism and casual thinking tends to emphasize maternal physiology and welfare. In part, this is due to the fact that the maternal organism is reasonably accessible for day to day observations and measurements which do not jeopardize the pregnancy. The ever changing complex interactions between the maternal and embryonic or foetal environments at and within the uterus do not lend themselves to long term studies. In part, the emphasis on maternal physiology has been due to the fact that removal or experimental death of the foetus alone permits the maternal physiology of pregnancy to continue until the placenta is delivered at term. This has been reported in rabbits, rats, mice, cats, hamsters, and monkeys but a critical perusal of data presented does not always lend confidence in the generalization. The concept that the foetus is a metabolic sponge albeit with interesting growth patterns, and acting as a passive partner in the gestation is not tenable and within recent years the embryo and foetus itself and maternal-foetal interactions have become proper subjects for investigation. These combined with continuing interest on the placenta, and the application of ingenious experimental approaches, has shifted the emphasis on the physiology of pregnancy in due part to the total uterine contents. As such the problems of embryonic mortality, welfare of the foetus, and hence of the newborn can, and has, come under more intense scrutiny. It is in this light that the subject will be discussed with emphasis being placed on certain critical times and processes which determine the reproductive potential of an animal. It is worth noting again that the prenatal loss of eggs, embryos and foetuses is a staggering one. The data pertaining to preand post-implantation mortality among domestic livestock, wild rabbits, domestic rabbits, mice and rats have been summarized by various workers. (Brambell 1948; Casida 1953; Perry 1954; Amoroso and David 1960; Hanly 1961; Harper 1964.) It is not as well known that the maximum fertility rate of fertile women, estimated at 80-90% at fertilization declines to 58 % at implantation and to 42% after the twelfth day post coitum. The precise figures are not crucial. The importance of the data is that significant prenatal mortality is a general biological phenomenon and occurs in monotocous and *polytocous species with reproductive cycles and placental types which differ greatly.