The conditions under which implantation is delayed in mice mated at the post-partum oestrus have been investigated. The critical litter size for the effect was shown to lie between three and nine pups per female. Delay of implantation was dependent on the presence of the litter until at least day 3 and the length of the delay could be significantly shortened by temporary litter removal on days 2 and 4. A study of the effect of parity showed that delayed implantation after post-partum mating continued to occur until the 4th litter. The incidence and length of implantation delay were reduced after five or more litters.
During suckling-induced delay of implantation the increase in uterine capillary permeability which occurs on day 4 of normal pregnancy was shown to be delayed to as late as day 14. The rate of development and time of entry into the uterus of zygotes was also retarded in comparison with normal pregnancy. This pattern resembled that seen after hypophysectomy on day 1 of normal pregnancy.