Loss of hairs in nursing dogs is usually associated with hormonal imbalance. This imbalance results from changes in hormonal levels during pregnancy and lactation. Dogs have 2 general phases of hair growth. One is growth phase and other is resting phase. During pregnancy, dogs are under extreme stress, this stress can cause early resting phase of hairs. This result in hair loss. This condition is not abnormal because hairs lost in clumps due to the fact that new hairs are coming and pushing older hairs. If you brush your nursing dog on daily basis then this problem will be solved early.