Hormonal Trends During Reproductive Stages and Ultrasonographic Monitoring of Gestational Age in British Shorthair Cats.

As seasonal animals with multiple estrus cycles, cats have unique breeding characteristics. However, the trends of key hormone changes during the estrous, pregnancy, and lactation periods are not well understood, and there is a lack of ultrasound-based formulas for predicting fetal development at different stages. This study aimed to elucidate the dynamic changes in key reproductive hormones and establish ultrasound-based methods to assess fetal development and predict gestational age in purebred British Shorthair cats (n=5). Plasma concentrations of estradiol (E₂), progesterone (P₄), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and melatonin (MT) were analyzed across estrous, pregnancy, and lactation. The correlations between ultrasound measurements and gestational age were analyzed using Pearson's correlation coefficient. Additionally, we used ultrasound measurements to develop methods for assessing fetal development and predicting gestational age. E₂ peaked at 230.8 used ultpmol/L during estrus and showed a secondary peak around day 15 post-mating; P4 remained at baseline (1.02 ± 0.58 nmol/L) during estrus but reached a peak of 123.11.11 daynmol/L around day 21 of pregnancy. FSH and LH showed characteristic fluctuations, with peak concentrations of 29.2 ± 6.19 IU/L and 43.3 ± 12.23ng/L, respectively. Gestational age correlated strongly with ultrasound measurements of fetal length (r > 0.90), biparietal diameter, and femur length (all P < 0.001). Regression equations for these parameters were developed, enabling precise gestational age predictions. Notably, fetal heart rate followed a characteristic pattern, stabilizing at ~250 bpm mid-pregnancy before declining near parturition. These findings advance the understanding of feline reproductive physiology, providing critical benchmarks for reproductive monitoring, improved veterinary care, and breeding management.