REVISED 1 Efficacy of transvaginal ultrasound - guided twin reduction in the mare by embryonic or 1 fetal

17 Transvaginal ultrasound-guided pregnancy reduction (TUGR) is a procedure described for the 18 management of twins post-fixation in the horse. Success rates are often disappointing but 19 reported to be more favourable for bilaterally-situated twins, and when intervention takes 20 place before day 35 of gestation. This study aimed to determine whether stabbing the 21 embryo/fetus rather than aspirating conceptus fluids improved the likelihood of success, 22 measured as the birth of a normal live singleton foal. Data from 103 TUGR interventions 23 were analysed by logistic regression analysis; method of treatment, relative conceptus 24 location (i.e. uni-versus bilateral) and stage of gestation were included as interdependent 25 factors that potentially influence the outcome. Overall, 34/103 (33%) TUGR interventions 26 resulted in a single live foal. There was no significant difference (p=0.14) in outcome between 27 TUGR based on fetal stabbing (12/28: 42.9%) versus fluid aspiration (22/75: 29.3%). There 28 was also no significant influence (p=0.11) of the conceptuses being located unilaterally 29 (19/65: 29.2%) versus bilaterally (15/38: 39.5%). However, TUGR was numerically more 30 successful (p=0.05) when performed ≤ day 35 of gestation (21/53: 39.6%), as opposed to > 31 day 35 (13/50: 26%). Day 45 may represent an even more critical time point since only 2 out 32 of 15 TUGRs (13.3%) performed beyond this day resulted in the birth of a live foal, compared 33 to 11/35 (31.4%) performed between days 36 and 45. While the numbers are low, this 34 suggests that TUGR is not the method of choice for reducing > day 45 twins. Four pregnancy 35 losses were recorded 1-7 months post-TUGR (4/38: 10.5%) and, while it is tempting to 36 attribute the losses to TUGR, this rate of late gestation pregnancy loss is normal. We conclude 37 that TUGR by fetal stabbing does not offer significant advantages over fluid aspiration. 38 However, TUGR should be performed before day 35 of gestation and considered primarily a 39 salvage procedure to be used when re-breeding is not a viable alternative. 40