Prediction of fetal age is best done within the first 40-60 days of gestation by the ultrasound characteristics of the developing embryo/fetus. The growth or elongation of a variety of fetal body structures have been assessed in an attempt to determine fetal age, including eye diameter, biparietal diameter, aortic root diameter, femur length, thoracic diameter, and heart rate. Some growth characteristics may also be influenced by the age of the mare and parity status within breeds, such that older mares may have smaller fetal aortic root diameter measurements than in young mares or that primiparous mares may have greater fetal eye measurements than in multiparous mares. Embryo characteristics are fairly uniform across horse types. This chapter tabulates the relationship between eye length and days before parturition for pony mares, and the relationship between eye length and days of gestation for light horses.
- Embryo characteristics
- Fetal age