Unplanned pregnancy is a public health concern in both developing and developed world. It is predispose women to maternal deaths and illness mainly through unsafe abortion and poor maternity care, and also it causes adverse social and health outcome for mother, children and family as whole. Unplanned pregnancy can have serious health, economic, psychological and social consequence for women, their children and families. The objectives of this study are to assess the prevalence and associated factors of unplanned pregnancy among pregnant mothers in Arerti town. Community based cross sectional study was done from March 18-March 22, 2018 GC. Out of 334 pregnant women in Arerti town, data were collected from 327(97.9%) pregnant women. The prevalence of unplanned pregnancy was 97(29.9%). Use of family planning methods [AOR= 3.845(2.023-7.307)] and educational status of pregnant women [AOR=12.472(1.571-99.011)] were factors significantly associated with unplanned pregnancy. According to this study over one third of pregnant women had unplanned pregnancy. We Recommend that there is an apparent need to provide information and counseling about each contraceptive methods by health extension and other health service worker in order to make informed choice and correct method use and strengthening family planning methods among client in need of service and encourage women's education.