25 Estimating Fetal Age Using Transabdominal Ultrasonic Measurements in Sheep Early in Gestation

Abstract

Abstract An increasingly common method to detect pregnancy in sheep is transabdominal ultrasonography. Research has shown that transabdominal ultrasonography can be accurately measured between days 33–60 of gestation. Additionally, few studies have investigated the use of ultrasonic measurements as predictors of fetal age via transabdominal ultrasound. A pilot study found that pregnancy detection can be performed as early as 31 days with an accuracy of 100% and reported that embryonic vesicle depth (EVD) could be utilized to estimate fetal age. The purpose of the present study was to further evaluate the ability of transabdominal ultrasonography measurements to predict fetal age. Estrus was synchronized in fifteen crossbred ewes, followed by natural mating over a 24h period. From weeks 3 to 9 of gestation, transabdominal ultrasonography (at 3.5 MHz) was performed twice weekly. During on farm ultrasonography, pregnancy diagnosis and fetal count were recorded, and ultrasonic images were recorded for subsequent measurement of EVD and embryonic vesicle length. Eleven out of 15 ewes were confirmed pregnant from our sample. Using the surveyreg procedure in SAS we predict day of gestation through the independent variables: average embryo vesicle depth (aveEVD), aveEVD2, average embryo vesicle length (aveEVL), and the number of fetuses. The model predicts 82% of the variation in gestational age can be explained by the independent variables. Parameter estimates for aveEVD (P < 0.0001) and aveEVL (P < 0.0286) are positive, indicating that every 1 cm increase in these measurements results in 8.9 and 1.3 day increases in gestational age, respectively. Holding other factors constant, we find embryonic vesicle depth increases at a decreasing rate (aveEVD2, P < 0.001). These results indicate gestational age can be predicted through two embryonic vesicle measurements using transabdominal ultrasonography.