When a pregnant mare needs labor induced—in the case of prepubic tendon rupture, for example—we need to know whether the foal is skeletally mature enough to survive. While veterinarians can predict fetal age/growth within a two-week range using femur, eye, and cranial measurements prior to 200 days of gestation, they’ve lacked precise tools for predicting a foal’s age/growth beyond that time point. New data suggest, however, that ultrasonic evaluation of the foal’s first phalanx (P1) late in gestation, beyond 240 days, can help assess fetal growth and bone development.

Catherine Renaudin, DVM, Dipl. ECAR (European College of Animal Reproduction), of UC Davis’ Department of Surgical and Radiological Sciences, explained how to use ultrasound to assess fetal bone maturation of the first phalanx (P1) during the 2021 American Association of Equine Practitioners (AAEP) Convention, held Dec. 4-8 in Nashville, Tennessee.

“There is a table that veterinarians can use in Quarter Horses that provides age prediction within two weeks if the mare is <200 days of gestation,” said Renaudin. “As pregnancy advances, then the prediction accuracy decreases. We therefore need better biometric parameters beyond 200 days of gestation.”

Renaudin said evaluating P1 in the equine fetus provides a noninvasive way to achieve this goal, as radiography isn’t practical. Specifically, Renaudin recommended measuring the length of the ossified portion of P1 using transrectal ultrasound of the dam. She suggested specialists create growth charts (of P1 length versus days of gestation) for Quarter Horse mares late in gestation (>240 days) based on this data.

“We know that after 240 days of gestation, normal equine fetuses should be in anterior presentation (with the head toward the cervix),” Renaudin said. “Therefore, the foal’s forelimbs are oftentimes extended near the cervix, making them evaluable via transrectal ultrasound.”

Renaudin and colleagues used this technique in eight healthy pregnant mares with known ovulation dates. They performed an ultrasound examination on each mare every two weeks beginning at 240 days of gestation. The mares were unsedated and restrained in stocks for the exams.

“P1 could be seen in utero in 43 of the 61 exams, or 70.5% of the time,” Renaudin said.

From the data, she found that P1 length correlated strongly with gestational age >240 days.

“When P1 length was equal to the width of the ultrasound image, most mares (7/8) were >300 days gestation,” Renaudin said.

Further, she and her team found that both the proximal and distal ossification centers (the area where bone forms) appeared between 277 (±2 weeks) and 303 days of gestation. The proximal ossification center of P1 did not close during the study period, while the distal ossification center closed between 313 (±2 weeks) and 333 days.

Renaudin said this technique provides practitioners with a new tool for assessing fetal growth/age after 240 days of gestation when prediction accuracy decreases. When the mare’s breeding date is unknown, the approach can help predict parturition date; however, it’s not absolute, because gestation length is highly variable (310-370 days) in horses.

“When breeding date is known, normal fetal growth is a sign of fetal well-being,” Renaudin said. “Thus, P1 length may be a better biometric parameter late in gestation compared to eye and cranium measurements.

“In addition, we now have a noninvasive way to assess fetal bone maturation,” she continued. “The appearance of P1 proximal and distal epiphyseal plates occurs slightly before 300 days, and the closure of the distal epiphyseal plate occurs during the last month of gestation. Time of the P1 secondary ossification centers’ appearance and epiphyseal closure could serve as a marker of bone maturation and help in the decision-making of not inducing parturition if complete bone maturation has not occurred.”