Feline Pregnancy Endocrinology: Hormonal Changes and Placental Function

Pregnancy in cats involves a distinctive endocrine pattern in which ovarian and placental hormones work together to support gestation. For practicing veterinarians, understanding when these hormonal sources change is particularly useful when interpreting reproductive findings, assessing pregnancy progression, and understanding why individual hormones have different clinical value. 

Cats are seasonal, polyestrus breeders, with reproductive activity influenced by light intensity and duration. Increasing daylight suppresses melatonin secretion from the pineal gland, promoting gonadotropin secretion and subsequent follicular growth¹. During estrus, serum estradiol reaches its maximum concentration¹. Mating may induce ovulation and corpus luteum formation, initiating the endocrine environment required for pregnancy. 

Progesterone: The Major Luteal Support 

Following mating, serum progesterone begins to rise around days 2–3. During the first half of pregnancy, corpora lutea are the principal source of progesterone. Serum progesterone concentrations are higher in pregnant cats than in non-pregnant cats, particularly during the first half of gestation². 

Progesterone reaches peak concentrations between approximately days 11 and 30 and subsequently declines gradually toward parturition¹. From the later part of gestation, the placenta takes over production of gestagens while the corpora lutea undergo regression³. 

This ovarian-to-placental transition is an important feature when considering progesterone physiology throughout feline gestation. 

Estradiol and Placental Steroid Production 

Estradiol falls rapidly during approximately the first five days after mating. In pregnant cats, however, concentrations subsequently increase until around day 15 and remain elevated until parturition. The placenta contributes to estradiol secretion during pregnancy^1,3. 

Importantly, individual variation in serum estradiol concentrations limits its usefulness for timing parturition. Therefore, estradiol values should not be interpreted in isolation when assessing proximity to delivery. 

The feline placenta also synthesizes progesterone. Placental progesterone concentrations increase toward late gestation, accompanied by expression of steroidogenic enzymes within placental tissue¹. 

Pregnancy-Specific Relaxin 

Relaxin provides another important endocrine marker of feline pregnancy. It is produced by the syncytiotrophoblast and is described as the only pregnancy-specific hormone in cats. Placental secretion begins from approximately day 20 of gestation, with the fetoplacental unit serving as the major source¹. 

Prolactin and Prostaglandins Near Parturition 

Prolactin is secreted by the anterior hypophysis. Its concentration increases toward parturition and remains elevated during lactation. 

Placental PGF2α secretion also increases toward term. This is reflected by increasing PGFM concentrations during the three weeks preceding parturition, with a functional rise during the final week⁴. This timing differs from dogs, in which the corresponding increase occurs much closer to parturition. 

Practical Clinical Insights 

For everyday feline reproductive practice: 

• Progesterone changes with gestational stage, reflecting the transition from luteal to placental gestagen production. 

• Relaxin is pregnancy-specific, with placental production beginning around day 20. 

• Estradiol is variable between individuals and is therefore unsuitable for accurately timing parturition. 

• PGFM increases before parturition, reflecting increasing placental PGF2α activity during late gestation. 

Recognizing these distinct endocrine patterns helps clinicians interpret reproductive findings according to gestational stage rather than relying on a single hormonal measurement. 

References 

1. 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 

2. Gültiken N, Yarim M, Aslan S, Gürler H, Yarim GF, Tuncay M, İnal S, Schäfer-Somi S. Expression of anti-Müllerian hormone and its type 2 receptor in the ovary of pregnant and cyclic domestic cats. Animals. 2022 Mar 30;12(7):877. https://www.mdpi.com/2076-2615/12/7/877 

3. 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 

4. Siemieniuch MJ, Jursza E, Szóstek AZ, Zschockelt L, Boos A, Kowalewski MP. Placental origin of prostaglandin F2α in the domestic cat. Mediators of inflammation. 2014;2014(1):364787. https://onlinelibrary.wiley.com/doi/pdf/10.1155/2014/364787