Causes of Mammary Enlargement in Dogs

When a bitch goes through a false pregnancy (also called pseudopregnancy or pseudocyesis), she thinks she has puppies. In one study, more than 60 percent of bitches had signs of false pregnancy after coming out of heat, whether they were bred or not. Mammary enlargement, with or without milk production, is a common sign of false pregnancy, but it’s not the only one. A bitch going through these hormonal changes eats more and gains weight. Some even display nesting behavior (protecting toys, other puppies, or even kittens) and later go on to strain as if trying to deliver puppies.

In most cases, bitches recover from a false pregnancy within a few weeks. If the mammary engorgement is severe, anti-inflammatory medication and hot packs may be required to relieve swelling and pain.

Another reason for mammary enlargement is mastitis, an infection of the mammary glands. This is painful, the milk is usually blood-tinged, and the bitch is clearly sick. This is treated with antibiotics and hot packs.

In older dogs, breast cancer must be considered if a mammary lump is detected. About 50 percent of these growths are malignant, meaning they will spread to other areas of the body. Research shows that the likelihood of mammary tumor formation is directly correlated with time of spaying. If a bitch is spayed before her first heat, her chance of developing breast cancer is less than 0.5 percent. The chance increases to 8 percent if she has just one heat before she is spayed. A bitch's risk rises to 27 percent if she has gone through two heats before spaying. Additional heat cycles do not increase the risk any further.