Ultrasonographic Monitoring of Pregnancy in Cats: A Gestational Timeline

Ultrasonography provides a practical way to follow the progression of feline pregnancy by identifying changes in gestational sacs, embryos, fetal membranes, placenta, and fetal organs at different stages. Because these structures develop progressively, recognizing their expected sonographic appearance can help the practicing veterinarian interpret pregnancy findings according to gestational age. 

Early Detection of Gestational Sacs 

The earliest recognition of feline gestational sacs has been described at approximately day 10. At this stage, the sacs appear as anechoic regions surrounded by hyperechoic borders and measure approximately 2 mm in diameter¹. 

During the embryonic phase, gestational sacs remain round until approximately day 20, after which they become slightly ovoid. Their diameter continues to increase, reaching approximately 35.7–37 mm between days 29 and 30¹. 

These progressive changes provide useful landmarks when assessing an early feline pregnancy. 

Identifying the Embryo and Cardiac Activity 

The embryo becomes detectable between approximately days 15 and 17 as a hyperechoic structure attached to the wall of the gestational chamber. Cardiac activity becomes visible at approximately day 16. By day 17, the embryo appears separated from the gestational chamber wall¹. 

Embryonic length also increases progressively¹: 

• Day 17: approximately 3.3 mm 

• Day 20: approximately 8.7 mm 

• Day 22: approximately 11.2 mm 

• Day 25: approximately 17.4 mm 

• Day 30: approximately 30.1 mm

Around day 20, fetal membranes become differentiated and surround the embryo, which is positioned centrally within the gestational sac¹. 

Assessing the Uterus and Placenta¹ 

The uterine wall can be differentiated into recognizable layers during pregnancy. The serosa appears as a thin, hyperechoic outer rim. The myometrium has intermediate echogenicity, ranging from hypoechoic to echoic, while the endometrium appears hypoechoic with multiple hyperechoic spots. 

The sonographic appearance of the placenta changes with fetal development. During the embryonic phase, placental layers are relatively homogeneous and difficult to distinguish. During the fetal phase, the placental layer becomes thicker and more structured as vascularization increases. It shows an inner and outer thin hyperechoic line with an inner hypoechoic line. 

Fetal membranes appear hypoechoic and become clearly visible after approximately day 30. These include the yolk sac, amnion and allantois. 

Fetal Development in the Second Half of Pregnancy 

As pregnancy progresses, fetal structures become increasingly identifiable. Internal organs can be differentiated from approximately day 30 onward. The layered structure of the intestine becomes visible later, between approximately days 52 and 56^1,2,3. 

During the second half of pregnancy, the biparietal diameter becomes greater than 15 mm. Uterine wall thickness has been described within a range of 2.4–6.8 mm¹. 

These developmental landmarks allow the veterinarian to follow the transition from a predominantly gestational-sac assessment in early pregnancy toward detailed fetal evaluation later in gestation. 

Key Takeaways 

• Gestational sacs may be recognizable from approximately day 10. 

• Embryos become detectable around days 15–17, with cardiac activity visible at approximately day 16. 

• Gestational sacs change from round to slightly ovoid around day 20. 

• Fetal membranes become well differentiated around day 20 and clearly visible after day 30. 

• Placental appearance changes as vascularization and fetal development progress. 

• Internal fetal organs can be differentiated from approximately day 30. 

• Intestinal layering becomes visible later, around days 52–56. 

• Ultrasonographic interpretation should account for the progressive developmental changes occurring throughout feline gestation. 

Understanding these gestational landmarks helps clinicians interpret feline pregnancy ultrasonographically and recognize whether the observed maternal and fetal structures correspond with the expected stage of development. 

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. Zambelli D, Prati F. Ultrasonography for pregnancy diagnosis and evaluation in queens. Theriogenology. 2006 Jul 1;66(1):135-44. https://doi.org/10.1016/j.theriogenology.2006.04.004 

3. Zambelli D, Caneppele B, Bassi S, Paladini C. Ultrasound aspects of fetal and extrafetal structures in pregnant cats. Journal of feline medicine and surgery. 2002 Jun;4(2):95-106. https://journals.sagepub.com/doi/pdf/10.1053/jfms.2001.0153