FACTORS CONCERNED IN THE DURATION OF PREGNANCY

Abstract

The author''s own work on the rabbit "lends no support to the view that the onset of labor is caused by changes in the fetus, senility of the placenta, or mechanical distention," but that pregnancy is under hormonal control. A gonadotropic factor producing ovulation during pregnancy may result in premature delivery, delay of parturition, or both in the same litter depending on the date and size of dosage. Considerable evidence supports the view that the corpora lutea are of chief importance in supporting pregnancy in the rabbit. Other workers have reported that the corpora lutea are essential throughout pregnancy in other animals, e.g., opossum and mouse, not essential in late pregnancy in the horse, guinea pig, cat and human. Removal of the ant. pituitary ends pregnancy in rabbits, but not in mice, rats, and guinea pigs if done late in the gestation period. Oxy-tocin is probably functional in normal deliveries. Considerable attention is given to the cyclic phenomena occurring during pregnancy; such as, repeated ovulations in the mare, follicular development and recession in the guinea pig, and periodic bleeding in the primates, all of which give some basis to the observation that normal gestation tends to be equal in length to a simple multiple of the sexual cycle.