Milk allergies are more common in babies than in older children or adults. In fact, approximately two to seven percent of babies experience milk allergies [source: BBC News]. A newborn with a milk allergy will typically start showing symptoms within the first few months. A baby can show signs of a milk allergy immediately after consuming cow's milk or a baby can have delayed onset of symptoms for up to seven or ten days after a feeding. Slower-onset milk allergies are more common in infancy. Most commercially available formulas contain cow's milk protein, and an infant with a milk allergy will react to the milk protein in the formula. Breastfed babies with a milk allergy react to milk protein that's found in the mother's breast milk [source: Kids Health].
A baby who cries frequently and is difficult to soothe, also known as colicky, may have a milk allergy. An infant reacts this way due to several symptoms related to a milk allergy. First, excessive gas can lead to significant discomfort for a baby. Second, a baby with a milk allergy may be malnourished and dehydrated. This can lead to failure to thrive, which is the inability to meet appropriate developmental growth milestones. Third, a milk allergy can cause eczema, a painful and irritating skin condition [source: Babies Online].
There are other important signs of a milk allergy in a baby, including persistent diarrhea and vomiting, and respiratory problems. Infants with milk allergies typically experience difficulty breathing, excessive mucus and persistent wheezing. If you notice these milk allergy signs in your baby, seek medical help. Prolonged milk ingestion can negatively affect your baby's growth and cognitive development. Typically, a nursing mother must eliminate milk from her diet or a baby must only ingest milk-free formula [source: Babies Online].