Morphometric changes of maternal fetal units due to their location within the uterine horns in the guinea pig

The objective of this research was to compare the post-mortem macroscopic morphometric characteristics of maternal-fetal units (MFUs) in each uterine horn (CU) and their relationship with progesterone (P4) concentration during the first 35 days of gestation in the guinea pig (Cavia porcellus). Twenty Peruvian line guinea pigs were used, with an average weight of 964.5 ± 33.99 g and a body condition score of 2.6 ± 0.11. The post-mortem evaluation was performed at 5 different times during gestation (days 15, 20, 25, 30 and 35), sacrificing 4 guinea pigs in each one. Blood samples were taken to measure P4 levels and then the reproductive system was removed, identifying the right horn (CUD) and left horn (CUI), which were divided into three sections: caudal, middle, cranial, to describe the site of embryonic implantation. The volume and weight of UMFs were determined, the placental diameter (DPL), gestational sac diameter (DSG) and crown-rump length (CR) were measured. The results showed a higher percentage of implantation in CUD (66.8 %) compared to CUI (33.2%) and a higher frequency of implantations in the caudal part of both horns (CpD = 32.5% and CpIZ = 22.1%). The volume of the UMFs showed a progressive growth, although it decreased on day 30, recovering again on day 35. A similar pattern was recorded in the development of DPL, DSG and CR. The concentration of P4 was 68.4 ± 3.41 ng·mL^-1 on day 15 and 63.7 ± 2.20 ng·mL^-1 on day 35. In conclusion, pregnancies occurred more frequently in the right horn, with implantations located in the caudal and middle positions of both horns. Embryonic development showed slow but progressive growth in the first 20 days, accelerating on day 30, driven by the functionality of the placenta.