Understanding the Canine Reproductive Cycle

As the weather warms and the days grow longer, predictably, veterinarians experience a rash of phone calls from panicked owners concerning the sudden onset of odd behaviors in their pets. Oftentimes, they report a decreased appetite, whining, crying, frequent urination or odd postural reactions. A spring virus? An injury? Infection? The first question asked by your veterinarian will likely be "Is your pet spayed?" If the answer is "No," then read on for a lesson in your pet's reproductive cycle.

Your dog's reproductive cycle is regulated by hormones produced both in the brain and the ovary. These hormones not only produce the changes in the reproductive organs needed for pregnancy but cause some dramatic departures in your pet's normal behavior as well. Hormones influence fertility and reproductive behavior in both the dog and the cat, although their heat and reproductive cycles vary by environmental and breeding behavior.

Canine Reproductive Cycle

There are basically three phases to the female dog's reproductive cycle: the follicular, luteal and quiescent phases. Each phase is under the influence of the dominant hormone produced. It is really an amazing system of timing and feedback from the brain to the ovary and back.

The dog's ovary is a collection of eggs in different stages of maturation. Each individual egg is actually enclosed in a tiny fluid filled sac called a follicle. A hormone produced by the pituitary gland at the base of the brain sends a signal for the eggs to develop within their follicles. This hormone is called follicle stimulating hormone (FSH) and causes the eggs to mature. Once the eggs have fully developed, they too have the ability to produce their own hormones. Follicles produce a hormone called estradiol 17B, and it is the dominant influence on the "follicular phase" of the reproductive cycle.

In the luteal phase, the pituitary gets busy again and secretes a hormone called luteinizing hormone(LH). The production of this hormone is triggered by the maturing follicles. LH acts on the follicle to stimulate ovulation, the time when the egg breaks through the follicle so that it can be fertilized. The egg then begins to travel down the Fallopian tube and the empty follicle undergoes its own physical change. It becomes a corpus luteum, the Latin name for "yellow body." It is named so because once the egg has ruptured, the follicle enlarges and turns an easily recognizable yellow color in contrast with the less mature follicles. The corpus luteum now has its own job to do, and that is to produce progesterone, the hormone of the luteal phase. It is in this phase that your dog exhibits mating and receptive behavior or signs of heat. You may see swelling of the vulva and bloody vaginal discharge that signal her fertility to potential mates.

Since dogs have multiple births, several eggs mature and are released at the same time. If fertilized, the eggs will implant on the uterine wall. This implantation results in a continued release of progesterone, the hormone responsible for maintaining pregnancy. If none of the eggs is fertilized, the body becomes aware that no eggs have implanted and progesterone is no longer released. This results in the quiescent or quiet phase. This stage may last several months, in which the dog will show no signs of sexual behavior.

Most dogs cycle twice a year in this manner, but it is safe to say that the reproductive cycle is unique to the individual. There will be variations in the number of heat cycles per year, the level of behavioral change, and interest in mating.


If your pet is bred and a pregnancy results, the average length of gestation is 63 days. This may vary a bit, but most people start to count the days of gestation from the day of the first breeding. If you only have a suspicion your pet may be pregnant and want to know for sure, your veterinarian can help make that determination.

During your pet's pregnancy, progesterone is the dominant hormone and is essential for maintaining the proper conditions in the uterus for the growth of the babies. About 10 days before puppies are born, the progesterone level falls and estrogen levels start to rise. As these hormone levels change, you will see physical and behavioral changes that signal labor and delivery. Your pet may seek seclusion, exhibit nesting behavior and her appetite may decrease. She may be restless. Most dogs pant. Mammary glands will distend and you will be able to express milk. A general guideline for monitoring the arrival of puppies is when your pet's rectal temperature falls below 100 degrees Fahrenheit, you can expect the onset of labor within 24 hours.

If the names of some of these hormones seem familiar to you it's because the very same hormones also regulate the human female reproductive cycle. While women are not constructed for litters the way dogs and cats are, the reproductive physiology isn't very much different. You may have more in common with your pet than you thought.