Late-gestation ewes are susceptible to ketonemia resulting from high energy requirement for fetal growth during the last few weeks of pregnancy. High lamb mortality is a possible consequence of effects of ketonemia on both ewes and lambs. Determining risk factors to ketonemia is a fundamental step to identify ewes at risk, in order to avoid losses caused by ketonemia. Serum β-hydroxybutyrate (BHBA) concentrations of 384 late-gestation ewe samples were determined. Physical examinations, including body condition, FAMACHA© and hygiene scoring, were performed. Udders and teeth were also examined. Fecal floatation was performed to detect gastrointestinal helminth eggs of the ewe fecal samples. General feeding management practices and season at sampling were recorded. Litter sizes were retrieved from lambing records. Factors associated with log serum BHBA concentration were determined using a linear mixed model, with flock and lambing groups as random effects. The mean serum BHBA concentration was 545.8 (±453.3) μmol/l. Ewes with a body condition score (BCS) of 2.5–3.5 had significantly lower log BHBA concentrations than ewes with a BCS of ≤2.0, by 19.7% (p = 0.035). Ewes with a BCS of >3.5 had a trend toward higher log BHBA concentrations compared to ewes with a BCS of 2.5–3.5. Ewes with a FAMACHA© score of 3 had significantly higher log BHBA concentrations than ewes with a FAMACHA© score of 1 or 2, by 12.1% (p = 0.049). Ewes in which gastrointestinal helminth eggs were detected had significantly higher log BHBA concentrations than ewes in which helminth eggs were not detected, by 12.3% (p = 0.040). An increased litter size was associated with higher log BHBA concentration (p ≤ 0.003), with the log BHBA concentrations of ewes having twins, triplets, and quadruplets or quintuplets were higher than those of ewes having singleton by 19.2%, 30.4%, and 85.2%, respectively. Season at sampling confounded the association between log BHBA concentration and FAMACHA© score, and therefore was retained in the final model even though it was not statistically significant. Intra-class correlation coefficients at the flock and lambing group levels were 0.14 and 0.32, respectively.