The objective of this study was to identify and explain the factors influencing the birth of underweight children in the city of Mbuji-Mayi. Methods: This is not a paired case-control study of births registered from 1 to June 30, 2015 in maternity hospitals in three health zones selected for this study, cases are all children born with low weight and witnesses are all children born with a normal weight is 2500 g and more. The significance level was set at p < 0.05.