Effect of pregnancy on lactation in mice

Albino mice were used to test the effect of simultaneous pregnancy on lactation in successive lactations. Weights of litters were used to estimate lactational performance by standardizing litters at eight pups on day 1 postpartum and weighing pups immediately after a normal nursing period on days 10, 14 and 20 of lactation. Eight dams were used in each of two treatments:lactating-nonpregnant and lactating-pregnant. Dams were sibling pairs for each treatment to reduce variation. Dams that were simultaneously pregnant and lactating had heavier litters on day 20 of the first lactation and on days 14 and 20 of the second lactation as compared to their lactating-nonpregnant siblings. The results imply that pregnancy provides a stimulus to concurrent and sucessive lactation in the mouse. This may be due to the effect of placental hormones on delaying mammary involution or on maintaining high milk synthesis past day 10 of lactation.