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In many rodents, including the rat and the mouse, implantation of the fertilized ovum fails to take place at the normal time if the female is lactating. The delay in implantation tends to be longer the larger the litter suckling, and pregnancy may be prolonged for several days. Observations carried out on normal mice showed that when pregnancy was of normal length in spite of a large suckling litter many of the young were stillborn and there was a reduction in litter size. The stillbirth rate of litters carried during a concurrent lactation was within normal limits only when the delay in implantation was at least 4 days.

Litter size at birth from delayed pregnancies was normal.

Lactation was unaffected by the presence of a second litter in utero.