Aging of the fetus using the fetal eye
Keywords: eye, fetus, equine, aging, ultrasoundThe fetal eye is seen here as a large anechogenic circle in the skull of the foal. Its size has been used to determine the approximate age of the fetus.
To estimate fetal age, the two largest diameters of the eye were measured and the sum of those results is plotted against a chart as shown. The chart was adapted from that originally published by McKinnon A.O. et al 1988. Note that the growth curve is curvilinear, not linear as has been suggested in earlier studies.
Newer data from Turner et al showed that the days before parturition (DBP) could be predicted with some assurance by measuring only the length of the fetal eye, centering the cursors over the lens to standardize measurements. Useful measurements could be obtained as early as 90 days of gestation. It was also emphasized that ponies had smaller eyes than those of horses at any given gestation and that allowance should be made for that fact when estimating fetal age.
Important: In cases of impending ventral rupture and neonatial isoerythrolysis etc, when foaling may need to be induced, these measurements are probably not accurate enough to decide on the correct time for induction. Milk electrolyte changes should be used instead.
In the author's experience (Lofstedt and Ireland 2000) in estimating the volume of sphere-like structures, it is more accurate to use the cubed value of the largest diameter of the structure than either the largest diameter, the sum of diameters, the circumference or the cross sectional area of the structure. Because volume is in effect, the actual size of a sphere-like structure, cubing a maximum diameter may be more useful than a simple diameter in future studies of this nature.
References:
Bucca S., Fogarty, U., Collins A. and Small V. 2005. Assessment of feto-placental well-being from the gestation to term: transrectal and transabdominal ultrasonographic features. Theriogenology. 64: 542-557
Turner, R. 2006. How to determine gestational age of an equine pregnancy in the field using transrectal ultrasonographic measurement of the fetal eye. Proc. AAEP 52: 250-255
Lofstedt R.M. and Ireland W.P. 2000 Measuring sphere-like structures using transtrectal ultrasonography. Vet Radiol Ultrasound. 41:178-180.
McKinnon A.O., Squires, E.L. and Pickett B.W. 1988. (No title) Eq. Reprod. Ultrason. Bulletin number four. Fort Collins, Colorado State University animal reproduction laboratory. 31-40.
