A matched case-control study of factors associated with retention of fetal membranes in dairy ewes in Southern Greece

We defined retention of fetal membranes (RFM) in dairy ewes as failure to expel the placenta within 6h after lambing the last lamb and designed a matched case-control study to identify factors that affect the risk of retention. For each ewe with RFM, the next ewe in the flock that lambed and expelled the placenta in <6h after lambing the last lamb was selected as control. Data analyzed included 92 pairs of ewes from 25 flocks comprising a total of 7275 ewes (median flocksize 270 ewes). Factors investigated for associations with RFM were induction of lambing, obstetrical assistance because of dystocia, parity, the number of liveborn lambs, the occurrence of stillbirth(s), the mean weight of the litter on the third day post-lambing and the occurrence of neonatal death in the litter. Conditional logistic regression indicated (1) that the risk of RFM increased linearly with increasing number of liveborn lambs and (2) that the risk of retention was 4-fold higher in ewes that received assistance at lambing than those that lambed normally.