Management of monozygotic twins in the mare

Abstract

In the horse, only a small number of monozygotic twins and triplets have been reported and commonly, the outcome is poor. As in man, an association between the occurrence of monozygotic twins and the use of assisted reproductive technologies (ART) has been suggested. In the mare, transfer of in vitro produced (IVP) embryos appears to increase the incidence of monozygotic twinning (1.6%; 4/254) compared to transfer of in vivo embryos (0%; 0/413) (Dijkstra et al. Equine Vet J. 2020; 52: 258-261). Due to the type of placental arrangement, these monozygotic twin pregnancies are difficult to reduce to a singleton using reduction techniques commonly applied to dizygotic twins. In this report, 8 cases of monozygotic twins are presented, all of which resulted from transfer of a single IVP embryo. Seven were presumed to be monochorionic diamniotic/diallantoic and one dichorionic diamniotic/diallantoic, based on membrane classification by ultrasound and/or post-expulsion examination. Similar to previously described cases, twin pregnancy only became apparent at 25 days of gestation, when two embryos with heartbeat were visualized. Depending on gestational stage, different twin reduction techniques were applied. In one case, only referred at day 40 of gestation andclassified as dichorionic diamniotic/diallantoic, a transvaginal ultrasound guided aspiration (TUGA) was performed. Reduction of two other twins was performed by per rectum compression of the fetal thorax at day 65 of gestation. In two other cases, cranio-cervical dislocation was performed at day 75-80 of gestation. Finally, transcutaneous ultrasound guided twin reduction was attempted at day 120 of gestation in one case. However, only one of the 6 treated monozygotic twins was successfully reduced, resulting in a live foal born following thorax compression at 65 days of gestation; all other cases resulted in death of both fetuses. In addition, two remaining cases (2/8) lost their pregnancy spontaneously at around 42 days and 8 months of gestation, whereby in the latter the presence of twins was not established at previous ultrasound examinations. Based on these limited cases, only one monozygotic twin was successfully reduced, using the thoracic compression method at day 65 of gestation. These results highlight the need for further investigation into the best reduction method, depending on the gestational stage and on how much the demise of one fetus affects the survival of its co- twin, which may depend on placental arrangement.