The length of the gestation period in mice is inversely related to litter size (Biggers, Curnow, Finn & McLaren, 1963). The effect is not a consequence of local crowding in the uterine horn, but is systemic. The total mass of the litter appears to be more important than the number of young (McLaren & Michie, 1963); but whether the influence regulating the onset of parturition emanates from the foetuses or placentae or both is unknown.
Reynolds (1949) and others have presented evidence that oestrogens can increase the excitability of the myometrium: it was therefore suggested by Biggers et al. (1963) that oestrogen produced by the placentae might sensitize the uterus to the action of oxytocin, and thus affect the time of onset of parturition. Evidence of oestrogenic activity is seen