Like most mammals, dogs have mammary glands regardless of their sex. In puppies, it is impossible to determine the sex of the dog by inspection of the nipples. Once a female puppy goes into her first heat or estrus cycle, you will notice her breasts enlarge, along with the usual signs of a swollen vulva and bloody discharge. This usually happens when she is between five and 12 months of age, depending on the breed. There are also some instances when enlargement is caused by a mammary disorder.
Female Puppies & Nipple Development
The mammary glands run on parallel rows from the chest area and extend to the groin. The female canine can have up to five sets of mammary glands identified as cranial thoracic, caudal thoracic, cranial abdominal, caudal abdominal and inguinal glands. The number of mammary glands may vary depending on the breed, with larger breeds usually having more teats than smaller breeds. Breasts are made up of a complex of glandular tissue and its corresponding nipple. Each nipple will have a number of ducts through which milk comes out during lactation and nursing.
As in all other mammals, the mammary glands of the female canine function as the vehicle for delivery of nutrition to newborn puppies. In the first 24 hours following whelping, they serve to pass on the mother's antibodies to her newborn puppies by secreting colostrum. In the first few weeks of life, colostrum gives the puppies immunity to various diseases, and greatly increases their chances for survival during the critical early weeks.
Causes of Normal Enlargement of the Nipples and Mammary Glands
Enlargement of the nipples and mammary glands happens as a response to the hormone estrogen. Levels of this hormone normally increase when a female dog goes into heat, reaching their peak a few days before ovulation and gradually decreasing as ovulation nears. It is not unusual for a dog's breasts to enlarge during, or right after, a heat cycle.
Breast enlargement is also one of the signs of pregnancy. By the fifth week of pregnancy, owners will notice their dog's nipples begin to darken and increase in size. Milk may be present at nine weeks, just a few days before whelping.
After giving birth, milk production increases to keep up with the demand from the litter. In between feedings, the owner will notice the dog's breasts become engorged as they fill up with milk.
Common Mammary Disorders
Some disorders may also cause enlargement of a dog's mammary glands. Pseudopregnancy, or false pregnancy, is a condition where the dog's body behaves as if it were pregnant, even if the dog was not bred. A possible effect of this condition is breast enlargement, and occasionally, milk production.
Another cause of enlarged breasts is mastitis, which refers to inflammation and infection of the mammary glands.
Mammary gland tumors, which can be benign or malignant, also cause breast enlargement in affected dogs.
Pseudopregnancy is a common occurrence in canines and usually does not require any treatment.
Mastitis may be treated simply with a warm compress and antibiotics.
On the other hand, mammary gland tumors require surgery to remove the affected glands and to perform a biopsy. Spaying is also recommended in order to prevent future occurrences. Chemotherapy may also be suggested, especially in aggressive tumors.