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Objectives To compare birth weight-to-placental weight (BW:PW) ratios between pregnancies complicated by gestational diabetes mellitus (GDM) and normoglycaemic pregnancies, and to evaluate the associations between BW:PW ratio, perinatal outcomes, and placental histopathologic features within the GDM group.Design A prospective cohort study.Setting A university hospital in Bangkok, Thailand.Participants A total of 200 women with GDM and 100 normoglycaemic controls.Outcome measures BW:PW ratios were calculated and compared between the two groups. Participants with GDM were stratified into three categories based on the BW:PW ratio percentiles: <10th, 10th–90th and >90th. Perinatal outcomes and placental histological abnormalities were analysed across these categories.Results Median BW:PW ratios were not significantly different between the GDM and normoglycaemic groups: 6.3 (IQR 5.6 to 6.9) versus 6.2 (IQR 5.6 to 6.8); p=0.399. Within the GDM cohort, the BW:PW ratio cut-offs corresponding to the 10th and 90th percentiles were 5.2 and 7.6, respectively. The prevalence of small-for-gestational-age (SGA) neonates differed significantly among the three BW:PW ratio groups: 14.3% (<10th), 1.3% (10th–90th) and 4.5% (>90th); p=0.004. Similarly, the prevalence of chorangiosis varied significantly across these groups (66.7%, 52.9% and 22.7%, respectively; p=0.009). A BW:PW ratio <10th percentile was associated with an increased risk of SGA neonates (adjusted OR (aOR) 9.94; 95% CI 1.14 to 86.80), while a ratio >90th percentile was associated with reduced odds of chorangiosis (aOR 0.35; 95% CI 0.11 to 0.85).Conclusions BW:PW ratios did not differ significantly between the GDM and normoglycaemic groups. However, in GDM pregnancies, extremes in the BW:PW ratio were associated with distinct perinatal and placental outcomes, indicating altered placental efficiency and potential clinical relevance.Trial registration number TCTR20211122001.