Abstract Background The prevalence of iron deficiency anaemia remains high in pregnant women and the situation may be worse for pregnant adolescents. This study aimed to comparatively analyse the trends and determinants of anaemia between adult and teenage pregnant women in rural Ghana. Methods A retrospective study design was employed. Data including primarily pregnancy history, haemoglobin levels and anaemia status were collected from the manual medical records of 1002 women stored in the repositories of two rural district hospitals in the Ashanti Region of Ghana over the years 2011–2015. Data was analysed using chi-square analysis, t-test, two-way ANOVA and binary logistic regression. Bivariate and multivariate analyses were also done to establish associations and predictors of anaemia. Results An overall drop in mean haemoglobin from 11.1 g/dl in 2011 to 10.6 g/dl in 2015 was observed for adults, while an overall increase from 9.4 g/dl to 10.2 g/dl occurred in teenagers. Further, anaemia prevalence declined at the 36th week of pregnancy, and from 2011 to 2015, for both teenagers and adults. Among factors tested for association with haemoglobin levels using bivariate and multivariate analyses, gestational age alone was significant (P = 0.028). Between-subject effects determined using 2-way ANOVA indicated year of pregnancy alone, as well as in combination with age group (F = 3.1, P = 0.019) significantly affected haemoglobin levels. From binary regression analysis, BMI (OR 0.967, 95% CI:0.936–0.999, P = 0.042), gestational age (OR 1.058, 95% CI:1.013–1.106, P = 0.011) and pregnancy year (OR-2012(0.402); 2013(0.53); 2014(0.569); 2015(0.817), 95% CI: 2012(0.256–0.631); 2013(0.338–0.829); 2014(0.366–0.886); 2015(0.501–1.333), P = 0.001) were found to be significant predictors of anaemia at first antenatal visit. Also, the trimester of reporting for antenatal care, specifically the second trimester (OR-0.261, 95% CI: 0.072–0.951, P = 0.042) and the pregnancy year (OR-2012(0.235); 2013(0.206); 2014(0.530); 2015(0.222), 95% CI: 2012(0.101–0.545); 2013(0.081–0.522); 2014(0.197–1.428); 2015(0.049–1.018), P = 0.003) were more significant at the 36th pregnancy week. Conclusions Although the trends observed were decreasing in both adults and teenagers in the years reported, anaemia levels remained high for each year in either group. Anaemia, therefore remains a major health problem, especially in the areas studied, and antenatal interventions need a second look to know what might make them more effective.