Real-time ultrasound measure of the fetal eye (vitreous body)
for estimation of parturition date in small ponies

RM Turner, SM McDonnell, and EM Feit
With technical assistance from EH Grogan and R Foglia

University of Pennsylvania School of Veterinary Medicine New Bolton Center, Kennett Square, PA 19348
1Ross School of Business, University of Michigan, Ann Arbor, MI 48109


     Ultrasonographic measurement of in utero fetal eyes has been used to estimate gestational age in horses, pigs, cattle, sheep, goats, dogs, cats (Kahn et al 1987; Kahn et al 1990, Chavez Moreno et al 1996), fallow deer (Lenz et al 1993), and humans (Goldstein et al 1998).  In horses, fetal eye measures have been published for light horse breeds, but not for smaller pony breeds.  Since foals and fetuses from small pony breeds are proportionately smaller than age-matched horse foals and fetuses, we hypothesized that horse fetal orbital measures would not be accurate for predicting the gestational ages of small pony fetuses.  Additionally, we hypothesized that once normal values had been established for small pony fetal orbits, these values could be used as a practical, on-farm procedure to estimate days to parturition in pregnant small pony mares.  To test these hypotheses, we used transrectal ultrasonography on pregnant small pony mares to measure fetal orbital size in utero at monthly intervals throughout gestation.  Twenty-three pony mares, ranging in age from 1 to 15 years, were examined once monthly during the gestations of their 2004 and 2005 foals.  These Shetland-type (100 to 250 kg) mares are maintained in a semi-feral pony herd for the study of reproductive physiology and behavior.  Eighteen pregnancies were studied in each year, with 13 of the 18 mares from 2004 included in 2005.  Nine of the pregnancies were primiparous in yearling fillies, 27 were multiparous (2nd to 10th gestation or greater).  For 2004 foals, examinations were conducted monthly from December 2003 until mares foaled (April through July of 2004).  For 2005 foals, examinations were conducted from August 2004 until mares foaled (March through July of 2005). Transrectal ultrasonographic examinations were performed on pregnant mares at pasture using either a Sonovet 2000 or a Medison SV600 veterinary ultrasound machine each equipped with a 7.5 mHz linear array transducer. The length (from sclera to sclera) and width (from retina to cornea) of the vitreous body were measured.  Resulting data included 3 to 9 monthly measures per pregnancy. For the 273 examinations in which fetal age was greater than 90 days, an eye was successfully isolated and measured in 248 instances (91%). Mixed-effects linear regression modeling was used to account for serial growth measures within pregnancy, repeated measurements across mares, and unbalanced study design. Independent variables evaluated included vitreous body length, vitreous body width, the ratio of length to width, parity, and mare height at the withers at parturition. Eye length was found to be the best predictor of days to foaling, with almost no additional predictive value of the many variables considered. Our resulting regression equation is: days to foaling = 264.44 – 0.21*(vitreous body length in mm)2. This study suggests that measure of the fetal eye is a practical on-farm veterinary procedure for estimating days to parturition in small ponies when breeding dates are not known.

This is a Dorothy Russell Havemeyer Foundation project.