Is it normal for mother dogs to lose their hair while nursing?

Yes.

Losing hair while nursing is a common and natural occurrence for mother dogs. During pregnancy and lactation, the drop in progesterone levels can lead to increased shedding. This is because the hormone progesterone, which is high during pregnancy, helps to keep hair follicles in place. Once progesterone levels decrease after giving birth, the hair follicles that were held in place by progesterone are released and shed.

Typically, the shedding during nursing is more noticeable in areas such as the belly and flanks. In most cases, the hair will begin to grow back once the puppies have been weaned and the mother dog's hormone levels return to normal. However, some dogs may experience more significant hair loss during nursing, and it's a good idea to consult a veterinarian if you are concerned about the level of shedding your mother dog is experiencing.