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Pregnancy Termination in Dogs and Cats
Revised: July 15, 2023
Published: October 28, 2003

Drawing of dogs with hearts
Illustration by Dr. Wendy Brooks

Sometimes nature takes an unwanted course. Maybe you kept putting off scheduling your female pet’s spay surgery, and before you knew it, she was in heat. Perhaps there was an especially industrious male in the neighborhood. Maybe your newly adopted female pet was assumed to have already been spayed, and, to your surprise, she wasn’t.

Accidents happen.

The situation described above is called mismating. Sometimes the actual deed is witnessed. Sometimes the pregnancy is discovered midterm. Your natural tendency may be to see if it can be undone right away before it gets too far, but unfortunately, hormonal drugs that can accomplish this carry an unacceptable rate of side effects. There is no safe "morning after" treatment (see below). The best choice is to wait until the pregnancy is confirmed and leave the decisions until that time. 

Your pet dog or cat can be tested for pregnancy with a simple blood test, similar to a home pregnancy test, after approximately 30 days of pregnancy. The test is called Witness Relaxin and detects one of the pregnancy hormones.

After Pregnancy is Confirmed, Review the Options

Have the Babies

This will be more work than you might think. Your pet will most likely be able to manage the birth and early care of the babies, but you will be in charge of finding homes for the offspring as well as the mom’s prenatal care.  You will need to know what the signs of trouble are during labor and how to care for your pet during pregnancy. We have information on both these subjects:

Before allowing this birth, please consider that there is an incredible pet overpopulation problem. We need more homes and fewer animals to solve this problem. This is your opportunity to be part of the solution.

Spay During Pregnancy

The spay surgery involves the removal of the uterus and ovaries. If the uterus is gravid (i.e., carrying developing young), the developing embryos are removed along with the uterus, and the pregnancy is terminated. Furthermore, the female cannot ever become pregnant again. If you do not intend to breed her in the future, this is probably the best option.

There is an increased risk to the female when she is spayed during pregnancy versus when she is spayed routinely. The blood vessels of her reproductive tract become huge and more difficult to tie off during pregnancy. The surgery takes longer, and there is usually an extra charge for this. Sometimes she must stay an extra day in the hospital or wear a bandage around her belly at home. The surgical scar will be much longer than it would be for a routine spay. The risk of excessive bleeding in surgery increases with the size of the dog and the stage of pregnancy.

Discuss the procedure with your veterinarian so that you understand what is involved and what to expect. Ideally, the female should be out of heat but not in advanced pregnancy.

Medical Abortion

If it is important that the female animal be bred in the future, the pregnancy can be terminated without sterilizing her. This entails the use of medications to end the pregnancy during the second trimester, about 30 to 40 days into the pregnancy. Typically, the female is hospitalized for 5 to 7 days for the procedure and returned to her owner in a non-pregnant state. It is helpful to know the breeding date as different medications work only during certain stages of pregnancy. Hospitalization is generally needed because some of the medications will be injectable, and there will be objectionable vaginal discharge that is undesirable at home.

The pituitary gland of the pregnant female secretes two important hormones: Prolactin and luteinizing hormone (affectionately termed LH). Both of these hormones nourish and sustain an ovarian structure of pregnancy called a corpus luteum, which in turn secretes progesterone, the hormone that directly maintains the pregnancy.

The pituitary gland of the pregnant female secretes two important hormones: Prolactin and luteinizing hormone (commonly called simply “LH”).  Both these hormones nourish and sustain an ovarian structure of pregnancy called a “corpus luteum,” which in turn secretes progesterone, which is the hormone that directly maintains the pregnancy. Disrupting the hormones breaks down the corpus luteum, and the pregnancy ends.

Medications used typically include something to disrupt prolactin secretion (either cabergoline or bromocriptine) in combination with prostaglandin (a hormone to induce uterine contractions and directly destroy the corpus luteum). Less cramping and vomiting is associated with cabergoline over bromocriptine, but the cost is substantially higher. Cabergoline is only recently available in the U.S.

Another protocol using dexamethasone, a steroid hormone used commonly for an assortment of medical problems, can be used to terminate pregnancy. High doses are used, making side effects common (excessive thirst, excessive urination, sometimes with incontinence, panting). A typical protocol involves 9 to 12 days of medication, does not require hospitalization, and involves minimal vaginal discharge if completed by Day 40 of pregnancy. Some experts feel this protocol is not as reliable as combinations of the other medications mentioned, while other experts prefer this one. The dexamethasone technique appears to be an option for dogs but not cats.

Your veterinarian will likely have a particular protocol that she has used before and feels is reliable and comfortable for the pet.

Some sort of pregnancy test after the abortion procedure is a good idea to be sure the procedure is effective. This kind of testing could be another Relaxin test, belly radiographs, blood progesterone levels, ultrasound, or any combination of these.

Warning: The Mismating Shot

Years ago, you could bring the mismated female dog to the veterinarian for an injection of estrogen within the first few days after the mismating. This treatment is usually not recommended anymore as the high doses of estrogen used predisposed the female to a life-threatening uterine infection called pyometra (up to 25% incidence in one study). Dangerous bone marrow suppression is also a possibility, plus the estrus and all the unpleasant vaginal discharge and attraction of males become prolonged. This treatment has been deemed too toxic and should not be requested.

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