So a team from the University of California, Davis, set out to establish a table (like doctors use in human medicine) with which they could accurately predict Quarter Horse fetal age using multiple biometric parameters. Catherine D. Renaudin, DVM, Dipl. ECAR, a veterinarian at the university, presented their findings at the 2020 American Association of Equine Practitioners virtual convention.

First, her team performed ultrasound exams on 10 healthy pregnant Quarter Horse and Thoroughbred mares with known ovulation dates and normal foals every two weeks from 100 days of gestation to parturition (foaling). Using these ultrasound images, they measured four fetal parameters—eye volume, cranium diameter, aorta diameter, and femur length—and generated a table with the predicted value for each variable from 90 to 350 days of gestation.

Next, said Renaudin, they challenged the table’s accuracy using 23 healthy, pregnant Quarter Horse mares with known ovulation dates and normal foals. An ultrasonographer blinded to the mares’ ovulation dates measured the four fetal parameters and used the table to determine each mare’s predicted day of gestation. Renaudin said they then assessed each biometric parameter’s accuracy and found they could predict gestational age and growth:

• Within two weeks at less than 200 days of gestation using femur length, cranium diameter, and eye volume;
• Within three weeks at 200-300 days gestation using cranium diameter and eye volume; and
• Within three weeks beyond 300 days gestation using eye volume.

Veterinarians can use these assessments and predictions to monitor foal health, said Renaudin. “If you know the ovulation or breeding date and if the fetus size seems to be normal based on the table, then there’s likely normal placental development, and that’s a sign of fetal well-being,” she explained. “If you find a very small-sized fetus compared to the table, you need to look for twins, growth retardation, or placental insufficiency and follow up with full ultrasound fetoplacental evaluation.”

If you don’t know the mare’s ovulation and breeding date (e.g., if a mare was purchased at auction with no history), you can measure these parameters to estimate age of gestation and parturition date.

“We know the best assessment will be during mid-gestation,” said Renaudin. “Your accuracy will be better than in late gestation. Remember, however, that you only have rough estimate of age prediction, because mares’ gestation length is very variable–between 310 and 374 days.”

While this study has provided a table of predicted measurements for aorta diameter, eye volume, cranium diameter, and femur length from 90 to 350 days of gestation in normal Quarter Horse fetuses, Renaudin said more research is needed to find better biometric parameters after 200 days of gestation.