BackgroundAnemia is a significant public health issue, accounting for 20–40% of maternal deaths. Despite the government's commitment and the interventions of various stakeholders, the magnitude and major risk factors of anemia remain unabated. Though there are few documented studies on anemia among pregnant women in eastern Ethiopia in general, in the study area in particular, some of the variables such as helminthics and history of caesarian section in relation to anemia need to be studied. As a result, the purpose of this study was to determine the magnitude of anemia and associated factors among pregnant women attending antenatal care in University Hospital in eastern Ethiopia.MethodsA facility-based cross-sectional study was conducted on a sample of 456 clients who were attending antenatal care in Hiwot Fana specialized university hospital from 01 to 30 June 2021. Systematic sampling was used to select the study participants. A pretested and structured interviewer-administered questionnaire and sample collection were used to collect the data. The data were coded, double-entered to Epi data version 3.1, cleaned, and exported to SPSS version 20 for analysis. Descriptive statistics were used to present frequency distributions. Variables with p-value < 0.25 during bivariate analysis were entered into the multivariate logistic regression models to control for all possible confounders to identify the factors associated with a magnitude of anemia. Odds ratio along with 95% CI were estimated to measure the strength of the association. The level of statistical significance was declared at a p-value of < 0.05.ResultA total of 456 participants were interviewed, with a response rate of 96.9%. The magnitude of anemia among pregnant women was 112 [(25.3%) 95%CI: (21.5–29.2%)], of which, 27 (6.10%), 36 (8.13%), and 49 (11.08%) had mild, moderate, and severe anemia, respectively. Birth interval < 2 years [AOR: 3.24, (95% CI: (1.88, 4.32)], number of children ≥2 [AOR: 2.54, (95% CI: (1.12, 4.64)], monthly income < 1,000 birr [AOR: 2.89, (95% CI: (1.31, 5.58)], third trimester pregnancy [AOR: 2.89, (95% CI: 4.86, 12.62)], and abnormal menstrual history [AOR: 2.28, (95% CI: (1.69, 5.24)] were the factors significantly associated with anemia.ConclusionAnemia among pregnant women was relatively high compared to previous studies. Birth intervals, number of children, history of menstrual disorder, monthly income, and trimester of pregnancy were all significantly associated with anemia in pregnant women.