BackgroundWomen’s undernutrition during pregnancy increases the risks and burdens of maternal and neonatal morbidity, death, and disability through its vicious cycles of irreversible intergenerational negative effects. Despite the high burden of maternal undernutrition during pregnancy in semi-pastoral communities of eastern Ethiopia, there is a paucity of information on its major risk factors. This study revealed determinants of acute undernutrition among pregnant women attending primary healthcare units in Chinaksen district in rural eastern Ethiopia.MethodA facility-based case-control study was conducted among 113 cases and 113 controls in Chinaksen district from February 01 to March 30, 2017. Data were entered using EpiData version 3.1 and analyzed using SPSS version 24. Multivariable logistic regression analyses conducted to identify significant determinants of acute undernutrition. Adjusted odds ratio (AOR) with a 95% confidence interval was used to report the strength of association and statistical significance declared atpvalue < 0.05.ResultsSixty (53.1%) of cases and 56 (49.6%) of controls were in the age group of 25-34 years and their mean ± SD age of cases and controls were 26.6 ± 5.7 and 28 ± 5.5 years, respectively. In this study, larger family size (AOR = 6.98, 95 CI [2.82–17.27]), lack of prenatal dietary advice (AOR = 3.68, 95% CI [1.67–8.00]), did not participate in a cooking demonstration (AOR = 5.41, 95% CI [2.39–12.24]), used substances (AOR = 3.65, 95% CI [1.30–10.23]), absence of basic latrine (AOR = 2.91, 95% CI [1.28–6.58]), low minimum dietary diversity of women (AOR = 2.48, 95% CI [1.20–5.12]), and household food insecurity (AOR = 3.06, 95% CI [1.44–6.51]) were significantly increased the odds of acute undernutrition among pregnant women.ConclusionsThe study revealed that living in crowded families, lack prenatal dietary advice, did not participate in cooking demonstrations, substances use; lack of toilet, low minimum dietary diversity, and household food insecurity were significant risk factors for acute undernutrition among pregnant women. Strengthening multi-sectoral approaches through improving dietary diversity/quality and food access/quantity would be essential to prevent, and reduce the risks, burdens, and impacts of maternal undernutrition during pregnancy.