Reason for performing study: Aged mares with endometrosis suffer higher rates of pregnancy loss than young mares, due to poor placental development. Reduced uterine blood supply may be one contributory factor.
Objectives: To measure uterine artery (UA) blood flow and other Doppler indices throughout pregnancy and compare placental and foal development in young mares and aged mares.
Methods: Thoroughbred mares were grouped according to age and endometrial biopsy score: 1) 6 young mares (mean age 7.3 years, Category I); 2) 6 aged mares (mean age 18.3 years, Category II). Vascular pathology was nil or mild except in one aged mare with moderate perivasculitis. Both UA were scanned fortnightly throughout pregnancy. Total blood flow volume (BFV, ml/min/kg bwt), peak systolic velocity (PSV, cm/s) and resistance index (RI) were determined by pulsed wave, Doppler ultrasound and UA diameter using B-mode. Mixed-effects regression analyses were used to relate vascular parameters with different predictive variables, whilst accounting for the multiple repeated measurements taken from individual horses through the duration of their pregnancies.
Results: PSV, RI and total BFV were best predicted by stage of pregnancy (P<0.001; r2>78%). The UA diameter was also associated with stage of pregnancy (P<0.001; r2= 87%) and was significantly greater in the gravid horn (P<0.001). There was a tendency for lower total BFV in older mares (P<0.05) and they delivered lighter foals than young mares (P<0.05). Gross placental morphometry was similar, but microscopic surface density of the microvilli was lower (P<0.02), in aged than young mares.
Conclusions and potential relevance: Increased uterine blood flow and decreased vascular resistance reflect fetal growth and development of the placental microcirculation. Older mares have poorer placental microvillus development and lighter foals with reduced UA blood flow. Poor uterine blood flow may be an important contributory factor for pregnancy loss in aged mares.