Fetal ultrasonography of the distal limb: a new tool to assess equine fetal age and bone development

Abstract

In Quarter Horses (QH), fetal age can be predicted within 2 weeks prior to 200 days of gestation via ultrasonography, but as pregnancy advances, accuracy decreases (Renaudin.et.al. Reprod Dom Anim. 2022;57: 743-753). Recently, fetal ultrasonographic measurement of the first phalanx (P1) length was introduced as a novel biometric parameter during late gestation, and the time of apparition and closure of P1’s epiphyses were found to be potential markers of bone maturation in utero (Renaudin.et.al. AAEP.2021;67:264-265). The aim of this study was to determine if bone characteristics of the second phalanx (P2), distal metacarpal 3 (MC3), proximal and distal sesamoid can predict fetal age. Ten healthy pregnant mares (9 QH and 1 Thoroughbred carrying a QH fetus) with known ovulation dates were used. Fetuses were examined by transrectal ultrasound every 2 weeks from 9 months of gestation until parturition. At each examination, the length of the ossified portion of P2’s diaphysis was measured, the presence or absence of P2’s and distal MC3’s epiphyses and the time of appearance of the proximal and distal sesamoid bones were documented. P2 length correlated strongly with days of gestation (P < 0.0001; r 2 = 0.75) and a correlation equation was established: y = 0.1413x -24.89 (y = predicted value of P2 length; x = fetal age). The P2 proximal epiphysis and MC3 distal epiphysis appeared on ultrasound between 285 and 306 days of gestation (mean: 294 days),and 260 and 294 days of gestation (mean: 268 days) respectively and did not close before parturition. P2 distal epiphysis was never imaged. Proximal and distal sesamoids were first observed between 286 and 306 days of gestation (mean: 295 days) and 300 and 329 days of gestation (mean: 317 days) respectively except for the smallest foal where they appeared at 331 and 335 days respectively. In conclusion, P2 length can be used in addition to P1 length as a biometric parameter in late gestation. Distal MC3’s, P1’s and P2’s epiphyses, and sesamoid bones diaphyses appear in a chronological order with MC3 distal epiphysis appearing first, P1’s second, P2’s third, proximal sesamoids fourth and distal sesamoid last. Distal limb ossification can be considered complete in QHs when the MC3 distal epiphysis, P1 and P2 proximal epiphysis and all sesamoid bones are present. Clinically, fetal ultrasonography of the distal limb may be used to assess fetal age and in the decision-making process of not inducing parturition if in utero bone development is not complete. This project was supported by the Center for Equine Health with funds provided by the State of California satellite wagering fund and contributions by private donors.