Feline Embryo Implantation and Early Placental Development

Pregnancy establishment in cats involves a sequence of precisely timed events between fertilization, embryo development, uterine preparation and implantation. For practicing veterinarians, understanding this early phase is useful when interpreting reproductive timing and recognizing how feline pregnancy differs from canine pregnancy. 

In cats, several early pregnancy events occur earlier than in dogs. The zygote enters the uterus approximately 5–6 days after coitus, while implantation is completed by around day 20¹. 

From Fertilization to the Uterus 

Following fertilization, the early feline embryo develops while passing through the oviduct. The zygote reaches the uterine lumen around days 5–6 after coitus, generally at the morula stage. During this passage, communication between the oviduct and developing embryo is thought to influence early development. 

The feline ovarian epithelium produces extracellular vesicles (EVs), including exosomes and microvesicles approximately 40–150 nm in diameter. These vesicles contain DNA, RNA, and proteins and can influence sperm function and fertilization. They can also be incorporated into the embryo, potentially affecting embryo development^1,2,3. 

Embryo Development and Uterine Preparation¹ 

After reaching the uterus around days 6–8 after fertilization, embryos migrate within and between the uterine horns. This spacing allows for more even distribution of embryos. By approximately day 7, most embryos have reached the blastocyst stage. 

Around day 10, the uterine surface epithelium becomes folded, and endometrial glands begin secreting. Through approximately day 12, estrogen and progesterone influence cellular and tissue changes that prepare the endometrium for embryo arrival. 

Estrogen promotes proliferation of endometrial and stromal cells, angiogenesis and increased expression of estrogen and progesterone receptors. Following corpus luteum formation, increasing progesterone suppresses proliferation and shifts the endometrium into its secretory phase. 

This hormonal preparation creates the uterine environment required for the next stages of embryo–endometrium interaction. 

Apposition, Adhesion and Implantation 

As the blastocyst develops, it loses the zona pellucida and expands. The embryo then progresses through apposition and adhesion. Cytoplasmic protrusions from the early embryo contact localized swellings and edematous regions of the endometrium, which become implantation sites¹. 

Around day 14, trophoblast cell columns begin penetrating the endometrium. Implantation is completed by approximately day 20, followed by the formation and differentiation of the zonary or girdle-shaped placenta¹. 

The feline placenta is endotheliochorial. Its gross arrangement can also differ from that seen in dogs. An incomplete zonary placenta was identified in 62.5% of pregnant cats in one set of observations, located distal to the umbilical cord insertion⁴. 

Decidualization and Uterine Receptivity 

Decidualization involves morphological and functional changes in endometrial and stromal cells, including proliferation, differentiation and expression of growth factors and hormone receptors. 

In feline endometrial and stromal cells, progesterone and estrogen produced morphological decidualization. However, some markers, including VEGF and the progesterone receptor, were not upregulated⁵. 

The precise mechanisms controlling feline decidualization and fetal–maternal communication remain incompletely characterized. 

Key Takeaways 

• Feline embryos enter the uterus earlier than in dogs, around days 5–6 after coitus. 

• Most embryos reach the blastocyst stage by approximately day 7. 

• Embryo spacing occurs within and between uterine horns after uterine entry. 

• Estrogen and progesterone coordinate endometrial changes before implantation. 

• Trophoblast penetration begins around day 14, with implantation completed by approximately day 20. 

• Feline placentation is zonary and endotheliochorial, with some cats showing an incomplete zonary arrangement. 

• Decidualization occurs under hormonal influence, but feline-specific mechanisms of uterine receptivity and embryo–maternal communication remain incompletely defined. 

Understanding this sequence helps clinicians place early reproductive events within the correct gestational timeframe and appreciate the important species-specific differences between feline and canine pregnancy. 

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. Harris EA, Stephens KK, Winuthayanon W. Extracellular vesicles and the oviduct function. International Journal of Molecular Sciences. 2020 Nov 5;21(21):8280. https://www.mdpi.com/1422-0067/21/21/8280 

3. Ferraz MD, Carothers A, Dahal R, Noonan MJ, Songsasen N. Oviductal extracellular vesicles interact with the spermatozoon’s head and mid-piece and improves its motility and fertilizing ability in the domestic cat. Scientific Reports. 2019 Jul 1;9(1):9484. https://www.nature.com/articles/s41598-019-45857-x.pdf 

4. Miglino MA, Ambrósio CE, dos Santos Martins D, Wenceslau CV, Pfarrer C, Leiser R. The carnivore pregnancy: the development of the embryo and fetal membranes. Theriogenology. 2006 Oct 1;66(6-7):1699-702. https://www.researchgate.net/profile/Carlos-Ambrosio/publication/7216757 

5. Wilsterman K, Bao X, Estrada AD, Comizzoli P, Bentley GE. Sex steroids influence organizational but not functional decidualization of feline endometrial cells in a 3D culture system. Biology of Reproduction. 2019 Nov 21;101(5):906-15. https://academic.oup.com/biolreprod/article-pdf/101/5/906/31137826/ioz145.pdf