Animal Reproductive Cycle


Dog Estrus Cycle

The estrus cycle of the dog is unique among domestic animals in several aspects:

The estrus (“Heat”) cycle of the bitch starts with swelling of the vulva and a blood tinged discharge that lasts up to 9 days (called Proestrus). During this time, the dog cannot become pregnant. This bleeding is sometimes likened to menstrual bleeding in the primate but it is actually entirely different.

After the bleeding stops, true estrus starts and lasts an average of 7 days but may last two weeks or more during which time the bitch is fertile and can become pregnant.

The bitch is unique in that in the absence of pregnancy, a condition of pseudopregnancy usually develops. In addition to the psychological changes usually associated with pregnancy, this may also lead to uterine and mammary gland development similar to pregnancy.

After a heat cycle, whether pregnancy or pseudopregnancy ensues, the bitch will not come back into heat for 6 months or more.

Sexual Maturity (Puberty)

Puberty occurs at 6 to 12 months of age in the dog, earlier in small breeds and later in larger breeds. Kenneled animals reach puberty later than free roaming animals. The good time to spay your dog is just before the first heat.

Litter Size

Litter size is extremely variable, especially between breeds. Some toy breeds may have litters of 1 to 3 puppies, whereas larger bitches may have litters of 10 to 15. The average for all breeds is 5 to 8 puppies.

Suppression of Estrus

Compounds are available for birth control in the dog: Megesterol Acetate (Ovaban, Schering) has been approved by the FDA; treatment is recommended only if begun for the first 3 days or proestrus (as soon as bleeding starts) or between heat cycles. Mibolerone (Cheque, Upjohn) is administered orally once daily for up to 24 months to suppress estrus.

Breeding Time

Female dogs will not stand for breeding until estrus is present. Vaginal smears can be examined under a microscope to determine the appropriate breeding time. Ovulation occurs on the first day of estrus, but the eggs need 2 to 3 days in the uterine tubes to become fertilizable. Canine sperm may live 6 to 10 days in the uterus. For a practical approach that takes advantage of the previous data, breed as soon as the bitch will stand and again 3 days later.

During the mating process, the male dog will ejaculate within 15 to 30 seconds after intromission. Dismounting soon follows but thereafter the penis will be “locked” in the vagina of the bitch for 10 to 30 minutes. The pair should be left undisturbed during this period to avoid injury to the genitalia.


The breeding of a bitch without the owners knowledge is a common problem. The therapy of choice is to perform an ovariohysterectomy (spay). If this is for some reason unacceptable, medical termination of a pregnancy may be considered. The most commonly used medication is an estrogen compound that needs to be given within 3 days of the mating. Several serious side affects are possible that make this practice dangerous to the pet and therefore undesirable. Pyometra (severe uterine infection) has been noticed in as high as 25% of the dogs given this medication. Estrogens also cause fatal bone marrow suppression in a significant number of dogs treated for miss-mating. Newer drugs given at the time pregnancy is diagnosed called Prostaglandins can be given but are not approved by the FDA for use in dogs. See below for protocols.


Facts about The Reproductive Cycle of the Domestic Cat— The domestic female cat (queen) has one extended breeding season a year, lasting from January to July, which may permit two litters. The queen will not ovulate without the physical act of breeding (copulation) or artificial stimulation of the cervix. This is Nature’s way of holding the ova (eggs) in limbo until the female and male come together. It is an efficient way to prevent ova from being shed and wasted in the absence of a male. When animals live under wild conditions, the male and female may not come into contact for several days at a time. Thus, Nature has provided a mechanism to discourage ovulation for several days, awaiting the arrival of the male. Other characteristics of the feline reproductive cycle are discussed below.

Sexual Maturity (Puberty)

The female reaches puberty at 7 to 12 months of age although it may be as early as 5 months. The male reaches puberty a month or two later than the female.

Breeding Season

The female cat will demonstrate estrus (“heat”) and sexual receptivity by a series of behavior changes. Physical activity such as pacing, rubbing against objects or the owner, and showing “affection” to the owner. Other actions include rolling on the floor, purring, squirming, stretching and generally showing a playful attitude. The cat may also have a lack of appetite and appear ill to the owner at this time. These activities are stimulatory to the male and attract his interest. When the male cat approaches, the female ceases her rolling and rubbing reactions and assumes the estrous posture which consists of dropping the forelimbs, resting the chest and abdomen on the floor and elevating the rear quarters, and treading with the back feet (this same reaction can be elicited by rubbing your fingers down the back of a queen in heat). The male cat will mount by grasping the neck of the female. Copulation will last only 1 to 3 minutes after which the female emits a loud cry (“coital cry”) and starts a violent rolling action, (“after-reaction”) and licking of the genitalia which may last for 15 to 20 minutes.

Estrous Termination

The estrous behavior of the queen can be very annoying to the owner of the cat and the veterinarian is sometimes asked to help in this situation. If the female is not bred, the estrous period will last 5 to 10 days, then stop for 2 to 3 weeks, then start all over again. This cycle will repeat until the female is bred or until the breeding season ends. Treatments include ovariohysterectomy (spay), sham mating, medical termination. Spay is the treatment of choice if the cat is not to be used for breeding purposes. A sham mating (artificial stimulation of the genital

tract of the female) may be done by your veterinarian. This will start the cat into a false pregnancy that will last 30 to 50 days. Medical termination can be done effectively but is not approved by the FDA and has some risks.

Male Sexual Activity

During the breeding season, the behavior of the male changes drastically. The male becomes aggressive and roams great distances, particularly at night, since cats are nocturnal. If a female in the neighborhood is in heat, the male can detect this and may guard the female for days or weeks, thereby protecting his “rights” by eternal vigilance. Sometimes several males may be in such intense competition that none can approach her. This may lead to destructive attacks by males on each other.


An infrequent problem is the misalliance of a female cat with an inappropriate male. Pregnancy can be terminated by spay or medically. The medical therapy can be done within 40 hours of breeding but it is not without risks. See discussions about miss-mating in dogs.

Pseudopregnancy (False Pregnancy)

In the event of a sterile mating or mechanical stimulation of the cervix, ovulation will occur but the eggs will not be fertilized. The uterus assumes the eggs were fertilized and prepares for pregnancy. During this period, lasting 30 to 50 days, the cat will respond in a manner characteristic of early pregnancy. Rarely is this a problem that needs therapy.


Clinical Problems, Presentations and Protocols

Abortion – Spontaneous – Cats

Presented with vaginal discharge and history of being pregnant but not at end of pregnancy term. This is a typical presentation for spontaneous abortion. The fetuses are not at a point in their development that they can sustain life outside the uterus. Possible causes include infectious diseases such as viruses, bacteria or protozoan parasites. Other causes include fetal malposition or fetal defects as well as uterine disease such as poor hormonal control, torsion, or toxic reaction.

Diagnostic Recommendations

Diagnostic Ultrasound (uterus fetus)Ultrasound of the reproductive tract can help determine number of fetuses as well as fetal viability. It can help us to predict if surgical intervention is necessary with this problem.

Radiographic analysisX-rays of the abdomen can detect fetuses that are fare enough along to have calcium in their bones. An x-ray can be used to determine the number of fetuses as well as some indication of viability.

Sick Cat Profile— This is an extensive laboratory analysis of the blood to determine internal organ function. Information regarding the status of infection, electrolyte balance and blood cell counts are needed to fully evaluate this severe case. This blood test also involves testing for the major viral and infectious diseases of cats including FeLV, FIV and FIA.

Therapeutic Recommendations

Amoxicillin trihydrate Injection (Amoxi Inject® [Pfizer]) 100mg/ml – 6-20 mg/kg SQ, IM q12h Amoxicillin is a potentated penicillin that has a broad spectrum of activity and is a good first line antibiotic.

Prostaglandin F2-alpha (Lutalyse, Upjohn) (Cat)— 0.25 mg/kg SQ q24h for up to 5 days promotes opening of the cervix and evacuation of the uterus through local mechanisms that stimulate uterine contractions.

IV Fluid Therapy— An IV catheter is placed and fluids are given to correct for losses during this process and to help maintain blood pressure. Antibiotics are given as well.

Additional Information

Until a diagnosis is made regarding the underlying problem, this cat has to be suspect of a contagious disease that may infect other pregnant queens or subsequent tom cats. Please have the hair clipped around the vulva so that close observation of discharges can be easily done. This condition frequently results in infections of the uterus or retained placentas and rechecks are important. It is not normally possible to determine the likelihood of uncomplicated subsequent pregnancies. If this animal is not to be used for breeding, we recommend ovariohysterectomy. If this is a valuable breeding animal, we recommend prostaglandin therapy as well as serial ultrasound evaluations and possible hysterotomy and flushing. Please watch the litter box for bloody uterine discharges, and Watch for high fever, lethargy, or not eating. Watch the mammary glands for swelling and failure to regress.

Abortion – Spontaneous – Dogs

Presented with vaginal discharge and history of being pregnant but not at end of pregnancy term. This is a typical presentation for spontaneous abortion. The fetuses are not at a point in their development that they can sustain life outside the uterus. Possible causes include infectious diseases such as viruses, bacteria or protozoan parasites. Other causes include fetal malposition or fetal defects as well as uterine disease such as poor hormonal control, torsion, or toxic reaction.