Objective <p>The timing of anaemia onset during pregnancy plays a critical role in determining its impact on neonatal outcomes. However, research on trimester-specific anaemia trends and their implications for adverse newborn outcomes has been inconclusive. We investigated trimester-specific trends in gestational anaemia and their associations with preterm birth (PTB) and low birth weight (LBW) in Ghana.</p> Methods <p>We randomly selected 389 postnatal mothers from selected healthcare facilities in the Tamale Metropolitan Area to conduct a retrospective study. Haemoglobin levels and birth data were electronically extracted from maternal health records. Trimester-specific anaemia trends were analysed, and associations with PTB and LBW were assessed using logistic regression models.</p> Results <p>More than half of the women (55.5%) experienced anaemia at some point during pregnancy, with a temporal trend increasing from the first (20.6%) to the second trimester (32.7%), followed by a decrease in the final trimester (6.7%). A significant proportion of women without anaemia in the first trimester developed anaemia in subsequent trimesters, with the highest proportion in the second trimester (37.9%). Despite corrections, a small proportion of women (2.5%) experienced a relapse of anaemia. Anaemia in the first trimester, but not in later trimesters, was significantly associated with increased odds of low birth weight (aOR = 3.25, 95% CI 1.35, 7.82, <i>p</i> = 0.008). Maternal anaemia in any of the trimesters did not affect the likelihood of preterm birth.</p> Conclusion <p>Anaemia is prevalent during pregnancy, with both its prevalence and impact varying by trimester. Anaemia during the first trimester increased the odds of low birth weight. Early screening and targeted interventions during the first trimester are crucial for mitigating adverse neonatal outcomes associated with anaemia.</p>