The Physiologically Immature Puppy
An immature puppy is at a distinct disadvantage because of his low birth weight and lack of muscle mass and subcutaneous fat. This pup may be unable to breathe deeply, nurse effectively, and maintain body warmth. His liver-brain ratio may be less than 1.5:1. His birth weight will be 25 percent below that of his littermates. Such a puppy is likely to be crowded out by his brothers and sisters and forced to nurse at the least productive nipples.
A common cause of immaturity is inadequate growth while in the uterus. The fault may be placental insufficiency, perhaps due to overcrowding or a disadvantageous placement of a placenta in the wall of the uterus. These puppies are immature because of their development rather than their age, and are at risk of developing cardiopulmonary syndrome.