Evaluation of equine fetal growth from day 100 of gestation to parturition by ultrasonography.

Transrectal and transabdominal ultrasonography were performed on normal pregnant mares (n=10) at 2 week intervals from day 100 of gestation to parturition to evaluate fetal growth. Several fetal anatomical regions (head, eye, aorta, abdomen, rib, gonad, kidney and femur) were imaged and measured using standardized scan plans. The results of these analyses indicate that all of the biometric parameters correlate strongly with the day of gestation. Growth charts were developed, which demonstrate that the following variables have linear relationships with the day of gestation on which they were measured: aortic systolic diameter, biparietal diameter, approximate eye volume, femur length and kidney cross-sectional area. The linear regression equation across days was developed for aortic, systolic and biparietal diameter, approximate eye volume, femur length and kidney cross-sectional area, thus allowing assessment of normal equine fetal development after day 100 of gestation. This non-invasive method can be used to estimate fetal age if mating or ovulation dates are unknown, provided the fetus is developing normally. The use of transrectal and transabdominal ultrasonography, as well as different probe frequencies (5.0 or 3.5 MHz), to measure different biometric parameters during gestation is reported.