The Effect of Suckling on the Duration of Pregnancy in the Rat (Wistar Albino)

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Abstract
It is an established fact that in the mouse and rat ovulation normally takes place between 12 and 32 hours after parturition (Long and Evans, 1922). Only rarely, however, does a pregnancy result from a mating at this time if the mother is suckling a litter (Kirkham, 1916), and in those instances where a second pregnancy does ensue it is of longer duration than is the case in the normal non-suckling animal. Daniel (1910) was of the opinion that an exact relationship existed between the number of young suckled and the extent by which gestation was prolonged. The absence of any such correlation was demonstrated by Kirkham (1916), who also investigated the cause of the prolongation of pregnancy in such cases. This was found to be due to a delay in implantation of the ova released at the post-partum ovulation, a delay which he ascribed to the inhibitory action of the mammary glands upon the uterus; he recognised that this activity was modified by the metabolic idiosyncrasies of the individual.