Unplanned and unwanted mating of cats and dogs is a common concern. Pregnancy can be completely prevented or terminated by ovariohysterectomy. Sixty percent of misbred bitches do not conceive, so confirmation of an undesired pregnancy is advised before proceeding. Postcoital douches are of no value in preventing unwanted pregnancy. Although injectable estrogens, when administered appropriately, can prevent pregnancy, their use involves great risk for serious side effects, including pyometra and potentially fatal bone marrow suppression, and they are not advised. They must be administered soon after copulation, before potentially fertilized ova reach the uterus. Oral estrogens given during diestrus greatly increase the risk of pyometra, are unreliable in terminating pregnancy, and are not advised.

Safe and effective termination of pregnancy is possible in both the bitch and queen by the administration of prostaglandin F_2α (natural hormone) dosed at 0.1 mg/kg, SC, tid for 48 hr followed by 0.2 mg/kg, SC, tid to effect (until all fetuses are evacuated as confirmed by ultrasonography). Treatment times can reach 14 days. In the bitch, treatment time can be reduced (usually by 48 hr) by the concurrent administration of prostaglandin E (misoprostol) intravaginally at 1–3 μg/kg, sid. The adverse effects of prostaglandins at this dosage (panting, trembling, nausea, and diarrhea) are mild and transient. The therapeutic window for prostaglandins is narrow and doses should be calculated carefully. Synthetic prostaglandins (cloprostenol given at 1–3 μg/kg every 12–24 hr to effect) more specifically target the myometrium.

Pregnancy can also be reliably terminated in the bitch by administration of dexamethasone at 0.2 mg/kg, PO, bid to effect. The owner should be informed of the side effects of corticosteroid administration (eg, panting, polyuria, polydipsia).

Combination drug protocols (cabergoline 5 μg/kg, PO, divided every 24 hr for 10 days, cloprostenol 1 μg/kg given SC twice at days 28 and 32 after the LH surge) have been reported to terminate pregnancy reliably with minimal adverse effects.

Last full review/revision July 2011 by Autumn P. Davidson, DVM, MS, DACVIM