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Objective Fetal hypoxia and acidemia have been reported in pregestational diabetic pregnancies in relation to poor glycaemic control, but it is still uncertain whether this is the case in apparently well-controlled gestational diabetes.Population and methods Maternal arterial and umbilical venous and arterial blood samples were collected from 37 normal (N) and 38 pregnancies complicated by gestational diabetes (GDM) at the time of caesarean section.Main outcome measures Respiratory gases, acid-base balance, lactate and glucose concentrations were measured.Results Both fetal and placental weights were significantly increased in GDM compared to N pregnancies, despite similar gestational age. Maternal biochemical parameters were similar in N and GDM but GDM fetuses were significantly more hypoxic (O 2 saturation: N 63.2 ± 13.9; GDM 53.8 ± 14.6%, P < 0.01; O 2 content: N 5.5 ± 1.4; GDM 4.8 ± 1.2 mmol/l, P < 0.05). Glucose (N 3.4 ± 0.5, GDM 3.9 ± 1.2 mmol/l, P < 0.05) and lactate (N 1.32 ± 0.49; GDM 1.64 ± 0.75 mmol/l, P < 0.05) concentrations were significantly increased in the umbilical vein in GDM compared to N fetuses. Placental histology was consistent with altered villous morphology.Conclusions Our data indicate that fetuses from gestational diabetic mothers have increased umbilical glucose concentrations despite normal maternal glucose levels and a reduction in oxygen saturation and O 2 content together with increased lactate concentration, reflecting altered fetal metabolism. These data suggest that 'good maternal metabolic control' achieved by currently used methods of monitoring glucose control is not sufficient to ensure a normal oxygenation status and metabolic milieu for the fetus in GDM pregnancies.