There is no set definition of neonatology in puppies and kittens. In humans, the neonatal period is defined as the initial period of complete dependency on the mother because of incomplete neurological functions such as audio- and visual abilities and immature spinal functions. By extension, these functions begin to appear by two to three weeks of life in puppies and kittens, and thus this period should be considered the neonatal stage of pediatric life in our small animals.

The clinical presentation of the neonatal puppy or kitten is often non-specific. Most notably there will be a lack of weight gain or weight loss. Failure to suckle, diarrhea, lethargy or vocalization, and seizures are among the more common clinical signs. The problem list generally consists of hypothermia, hypoglycemia and dehydration, all of which need to be addressed immediately. Moist mucous membranes are present in an adequate state of hydration, but tacky to dry mucous membranes indicate 5-7% dehydration. At 10% dehydration, the mucous membranes are very dry and there is a noticeable decrease in skin elasticity.

At birth, red blood cell counts, hemoglobin concentration, and packed cell volume are similar to those measured in maternal blood. However all of these values decline rapidly during the first 3 weeks of life, as the fetal hemoglobin is replaced by adult hemoglobin resulting in reticulocytosis. Nucleated red blood cells may be present, and a leukocytosis, especially in kittens, is common as the fetus slips out of its sterile environment into a world of pathogens, to which the neonate's immature immune system needs to respond. Lymphocyte counts can be highly variable in neonates and testing may need to be repeated if unusually high counts are noted.

Many serum biochemical parameters are low at birth because liver and kidney are immature. While BUN is on the low end of normal in neonates, serum creatinine concentrations are consistently below adult normal concentrations. Similarly, serum bilirubin, total protein, albumin, and cholesterol concentrations are lower than adult ranges. Glucose regulation is poor in neonates; lower than adult concentrations are typical. Serum alkaline phosphatase (ALP) concentrations are slightly elevated in neonatal kittens reflecting the increase in the bone isoenzyme due to increased bone turnover (growth). In neonatal puppies, very highly elevated serum ALP and gamma glutamyl transferase (GGT) concentrations can be measured during the first two weeks of life if the puppy ingested colostrum, as it is high in ALP and GGT. In puppies without colostrum, serum ALP can be elevated up to three times the adult normal concentrations (bone turnover) and serum GGT is in the low range of normal adult serum concentrations.

Knowledge of differences between neonatal and adult blood cell counts and serum biochemical concentrations are essential to understanding disease processes and developing therapeutic strategies.