Pregnancy is a time when the body is under a lot of stress, which increases your dietary needs. Under nutrition is a worldwide health issue, especially among pregnant women. Malnutrition during pregnancy can result in miscarriages, fetal deaths during pregnancy, pre-term delivery, and maternal mortality for both the mother and her fetus. Therefore, this research aimed to assess the prevalence of under nutrition and associated factors among pregnant woman attending antenatal care services at public hospitals in west Ethiopia. To assess the prevalence of under nutrition and associated factors among pregnant women attending Antenatal Care service in Public Hospitals of western Ethiopia. Facility based cross-sectional study was conducted from April 10 to May 10, 2020 among 780 pregnant mothers. The study participants were selected by systematic random sampling methods from antenatal care clinics of the hospitals. Interviewer administered structured questionnaire was used to collect the data and Mid-upper arm circumference, height and weight were measured to determine the magnitude of under nutrition among the study participants. The data were entered to Epi Info version 7.2.3, and then exported to SPSS version 24 for analysis. Multivariable logistic regression was used to identify independent predictors considering adjusted odd ratio (AOR) at p-value ≤ 0.05 to measure the strength of association between dependent and independent variables. The prevalence of under nutrition among pregnant women was found to be 39.2% (95%CI: 35.7%, 42.6%). Rural residence [(AOR = 1.97, 95% CI: (1.24, 3.14)], substance use [(AOR: 3.33, 95% CI: (1.63, 6.81)], low dietary diversity of women [(AOR = 7.56, 95% CI: (4.96, 11.51)], mildly food insecure household [(AOR = 4.36, 95% CI: (2.36, 8.79)], moderately food insecure household [(AOR = 3.71, 95%CI: (1.54, 8.79), and severely food insecure household [(AOR = 6.96, 95% CI: (3.15, 15.42)] were factors significantly associated with under nutrition. The study showed that the prevalence of under nutrition is very high among pregnant women. Factors associated with under nutrition of pregnant women were rural residency, household food insecurity, dietary diversity and substance use. All concerned bodies should made efforts to reduce the risk of under nutrition by reducing substance use and improving household food security there by to increase women's dietary diversity.