Predicting Mare Delivery Dates: The Eyes Have It

Thanks to a University of Pennsylvania study, owners of small ponies can now predict their pregnant mares' delivery dates simply by looking into their unborn foals' eyes. During a two-year project, researchers in the university's School of Veterinary Medicine used ultrasound imaging to measure the length and width of 36 fetal foals' eyes to establish size reference ranges at various stages of gestation. The study is the first to examine fetal eye measures as a birth date predictor for small pony breeds.

According to one of the lead researchers, Regina M. Turner, DVM, PhD, she and her colleagues set out to find a simple way to determine delivery dates for mares whose breeding dates are uncertain. They chose to measure eyes because they are typically clearly visible and easy to find on transrectal ultrasound in fetal foals beyond two months of gestational age.

Fetal eye

Measuring the size of a fetus' eye via ultrasound can help predict foaling dates.

"We wanted to find a practical way to measure fetal age in the field with equipment most practitioners already have," said Turner, who is assistant professor in the school's Section of Reproductive Studies. "So, someone who has a small pony mare that is pregnant and her breeding date is unknown can have a vet perform an on-farm transrectal ultrasound, measure the foal's eyes, then use our graph to determine when the birth will occur."

Even so, ranges gleaned from the study can't eliminate all the guesswork from foaling predictions, Turner said. That's partly because the gestation window can normally vary by as many as 40 days.

"Our prediction range values were also very wide," she said. "In later gestation, to predict foaling date with 95% accuracy, the best you can do is give an approximately 60-day window of days before parturition. Nonetheless, this can still be quite helpful when no breeding dates are known."

The authors plan to revisit and update a 20-year-old study of light horse fetal eye measurements. Future studies might gather eye measurement data for large-breed horses and examine what fetal eye measurements can reveal about foals during problem pregnancies.

About the Author

Pat Raia

Pat Raia is a veteran journalist who enjoys covering equine welfare, industry, and news. In her spare time, she enjoys riding her Tennessee Walking Horse, Sonny.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More