Background: Maternal underweight is a challenging public health issues globally affecting women in most developing countries and consequently affect their newborns. Such information however is scantly in the study area and thus we examined the contribution of low dietary diversity score towards underweight among expectant mothers from rural Ethiopia.Methods: This study was based on data extracted from a large data set gathered from three rural villages from September 2017 to June 2018 in a district of North-Shewa Zone of Amhara Regional State which is 220 km north of Addis Ababa, the capital city of Ethiopia. Data on socio demographic characteristics food groups consumed and Mid-Upper Arm Circumference (MUAC) measurement were taken for all expectant mothers by trained health extension workers in accordance with relevant ethical guidelines and regulations. The collected data were entered into Epi-data software version 3.02 and analyzed using SPSS version 20. Statistical significance was affirmed at a p-value ≤ 0.05. Result: Of the 400 consented participants, 380 of them had adequate information and thus analyzed for their variety of food groups consumption and their MUAC measurement outcomes. The magnitude of underweight was 18.2% and the mean (±SD) dietary diversity score was 4.45(±1.32). interestingly, the relationship between poor dietary diversity score and underweight of the participants was significant (AOR= 2.4; 95% CI=1.05 to 5.3). Conclusion: Our study revealed a significant relationship between low dietary diversity score and underweight of expectant mothers. it is thus, essential to improve the dietary diversity and nutritional status of pregnant women through strengthening the ongoing productive safety net program and empower women to earn more income so that mothers would purchase variety of foods and ultimately break the intergeneration cycle of malnutrition.