Superregnancy in the Mouse

Abstract

1. Superovulation was induced by gonadotrophin treatment in adult and immature female mice. The uteri of treated females and of control, naturally ovulating females were examined at 7½ days post coitum by means of a laparotomy operation, and again at autopsy 18½ days post coitum. 2. The mating rate of the two older groups of treated immature females was significantly higher than that of either the two younger groups or the treated adults The proportion of mated females which became pregnant was low in the youngest group, but increased rapidly with age up to the control adult level. 3. The mean number of implants per uterine horn in treated females in which implantation occurred was about twice that found in untreated control females, and did not vary with age. On the other hand, the variation in the number of implants per uterine horn was significantly greater among adult treated than among immature treated females. 4. Total failure of pregnancy through death or disappearance of the embryos was most common among younger females with very large numbers of implants. Partial litter loss through death of individual embryos was greater in adult treated than in immature treated females. Combining partial and total litter loss, the gross mortality rate of embryos was independent of the age of the mother. 5. Death of individual embryos before day 10 was more common in treated than in untreated females, and more common in adult treated than in immature treated females. The incidence bore no relation to the total number of implants either per female or per uterine horn, did not affect the survival of the remaining implants, and was not reduced by progesterone treatment. 6. The death rate of individual embryos during the middle period of pregnancy did not significantly exceed the control level until the number in the horn rose above eight, at which point it increased abruptly in all treated groups. In the immature, but not in the mature, treated females the presence of more than eight implants in both horns was associated with still higher mortality. 7. Death of individual embryos during the later part of pregnancy occurred relatively rarely and was associated with crowding of the uterine horn. 8. In adult treated females which were allowed to go to term, the number of young born alive was inversely related to the number of embryos which had implanted. 9. Our results are discussed with reference to the possibility of artificially increasing the reproductive output of mice and other mammals.