Background Abnormal birthweights are critical public health challenges accountable for most non-communicable diseases and perinatal mortalities. Regardless of the myriad of mixed evidence on maternal factors responsible for abnormal birthweight globally, most of these findings are attained from urban and rural settings. This study serves as one of key pieces of evidence in view of the increasing prevalence of abnormal birthweight particularly in some parts of semi-rural Ghana. The study therefore aims to estimate prevalence of abnormal birthweight and identify some possible maternal risk factors for abnormal birthweight in Northern Ghana. Methods A retrospective cross-sectional study was conducted in Savelugu municipality from February-March 2022. A total of 356 mothers aged 16−46 years and having a child born during the last four weeks, were recruited as study participants. Data were collected from maternal and child health record books and through structured interviews. Bivariate (Chi-square and Wilcoxon rank-sum tests) and multivariate (logistic regression) analyses were employed to identify maternal factors for abnormal birthweight at 95% significance level. Results Prevalence rates of low birthweight and macrosomia were 22.2% and 8.7% respectively. Maternal anaemia in first trimester (aOR:4.041; 95%CI: 1.741−9.381) and third trimester (aOR:25.71; 95%CI: 8.245−80.18) of gestation were strong predictors for low birthweight. Mothers belonging to minority ethnic groups (aOR:0.089; 95%CI: 0.009−0.859); mothers who had ≥ 8 antenatal care visits (aOR:0.264; 95%CI: 0.112−0.622); and mothers having neonates whose birth length > 47.5cm (aOR:0.280; 95%CI: 0.117−0.670) had reduced odds for low birthweight. Alternatively, mothers with gestational weeks ≥ 42 had 9.713 times higher risk of giving birth to macrosomic neonates (95%CI: 2.127−44.34). Mothers were more likely to deliver macrosomic neonates through caesarian section (aOR:4.981; 95%CI: 1.498−16.57). Conclusion The prevalence rates of low birthweight and macrosomia were relatively high. Anaemia in the first and third trimesters, and advanced gestational and caesarian section delivery age were strong determinants of low birthweight and macrosomia respectively. Being minority ethnic group, frequency of antenatal care visits, and childbirth length reduced the risk of low-weight births. Hence, nutrition counseling, community health education, and promotion of lifestyle improvement coupled with strengthening of health service delivery are recommended interventions.