Hormonal Control of Pregnancy Termination in Cats

Pregnancy maintenance in cats depends strongly on coordinated hormonal support, particularly progesterone and prolactin. This endocrine dependence also explains why interference with ovarian function or hormone action can terminate pregnancy. For practicing veterinarians, understanding how these mechanisms vary with gestational stage is important when considering pharmacological or surgical approaches to pregnancy termination. 

Progesterone and the Maintenance of Pregnancy 

Progesterone is central to pregnancy maintenance. During the first half of gestation, it is produced primarily by the corpora lutea, while later in pregnancy, the placenta contributes to gestagen production as the corpora lutea regress¹. 

The importance of ovarian support varies according to gestational stage. When ovariectomy was performed on day 35, abortion occurred in all cats. At day 40, abortion occurred in four-fifths of cats; at day 45, in two-fifths; and at day 50, in three-fifths. In the remaining cats, normal delivery occurred on days 63–69 of pregnancy². In another set of observations, ovariectomy on day 45 resulted in abortion in all cats². 

These findings indicate that placental development does not necessarily make ovarian function dispensable at the same point in every pregnancy. 

Progesterone Receptor Blockade 

The importance of progesterone is also demonstrated by the effects of the antiprogesterone aglepristone. Two injections administered on days 21–22 or from days 35–38 onward resulted in abortion in all cats³. 

When treatment was administered later, on days 45 and 46, abortion occurred in four-sixths of the cats. Progesterone concentrations remained high during abortion but subsequently decreased toward day 55². Another treatment schedule, with two injections given around day 33.3 on average, resulted in abortion in 54 of 61 cats². 

These observations highlight that the biological effect of progesterone depends not simply on its circulating concentration, but also on progesterone receptor activity. 

Prostaglandin-Mediated Pregnancy Termination 

PGF2α can also interfere with pregnancy maintenance. Repeated administration of the synthetic PGF2α analogue cloprostenol from approximately day 21–22 onward did not consistently induce abortion. When administration began from day 35–38 onward, abortion occurred in one-seventh of cats in one set of observations and one-tenth in another^2,3. 

Natural PGF2α produced a more pronounced response. Administration of 2 mg on day 33 resulted in abortion in all cats despite normal placental function and progesterone concentrations before treatment². Similar findings were observed when natural PGF2α was administered after day 40². In these observations, injections produced a significant decrease in progesterone to basal concentrations within 24 h. 

Prolactin Suppression and Cabergoline 

From mid-gestation onward, prolactin contributes to luteotropic support. Cabergoline, a dopamine agonist, antagonizes this effect. Daily administration for five days from day 30 resulted in abortion in four-fifths of cats after progesterone decreased to below 1 ng/mL². 

Daily oral administration between days 34 and 42 until abortion resulted in abortion in all cats. Similarly, administration beginning between days 36 and 40.8 induced abortion in all animals, whereas treatment beginning at approximately day 48.5 induced premature parturition instead^2,4. 

Conclusion 

Pregnancy termination in cats can involve disruption of several interconnected endocrine pathways. Progesterone receptor blockade, suppression of prolactin support, interference with ovarian function and prostaglandin activity can each affect pregnancy maintenance, but their effects vary with gestational stage. 

For clinical decision-making, the timing of intervention is therefore particularly important. The available observations also indicate that progesterone concentration alone may not fully reflect the functional status of progesterone support, especially when receptor activity is pharmacologically altered. These endocrine mechanisms provide the basis for understanding why different approaches can produce abortion at some stages but premature parturition or inconsistent responses at others. 

References 

1. Siemieniuch MJ, Jursza E, Szostek AZ, Skarzynski DJ, Boos A, Kowalewski MP. Steroidogenic capacity of the placenta as a supplemental source of progesterone during pregnancy in domestic cats. Reproductive biology and endocrinology. 2012 Oct 30;10(1):89. https://link.springer.com/content/pdf/10.1186/1477-7827-10-89.pdf 

2. Schäfer-Somi S. Establishment and Maintenance of Feline Pregnancy—A Comprehensive Review. Animals. 2025 Apr 28;15(9):1249. https://www.mdpi.com/2076-2615/15/9/1249 

3. Mitacek MG, Bonaura MC, Praderio RG, Favre RN, de la Sota RL, Stornelli MA. Progesterone and ultrasonographic changes during aglepristone or cloprosternol treatment in queens at 21 to 22 or 35 to 38 days of pregnancy. Theriogenology. 2017 Jan 15;88:106-17. https://sedici.unlp.edu.ar/bitstream/handle/10915/104818/.pdf?sequence=1 

4. Erünal-Maral N, Aslan S, Findik M, Yüksel N, Handler J, Arbeiter K. Induction of abortion in queens by administration of cabergoline (Galastop™) solely or in combination with the PGF2α analogue Alfaprostol (Gabbrostim™). Theriogenology. 2004 May 1;61(7-8):1471-5. https://www.researchgate.net/profile/Nilguen-Gueltiken-2/publication/8663699