OBJECTIVE To assess the reports that maternal phosphoglucomutase-1 (PGM1) phenotype is highly related to macrosomia in diabetic pregnancy. This could be either a direct metabolic phenomenon, or the PGM1 locus could be a marker for a tightly linked gene involved in the maternal control of fetal growth. METHODS A comparative biochemical genetic study. METHODS A large diabetic pregnancy clinic. METHODS One hundred and fifty-two women who had diabetes during pregnancy, 136 being insulin dependent before pregnancy. Two hundred and thirty-six women without pre-existing medical or pregnancy complications who functioned as a control group. METHODS PGM1 phenotype was assessed by conventional electrophoresis and subgroups were examined using iso-electric focusing. RESULTS Standardised birthweight was corrected for sex, maternal parity and gestation confirmed in every case by early pregnancy ultrasound. Maternal diabetes control was assessed by glycosylated haemoglobin. RESULTS No differences were found in the observed phenotype frequencies for diabetics and control pregnant women. No association between PGM1 phenotype and macrosomia in diabetic pregnancy was found. PGM1 did not make a significant contribution to birthweight, standardised birthweight, length or ponderal index of the baby as assessed by multiple regression. CONCLUSIONS Our study of a larger number of insulin dependent diabetics in Scotland makes the claim that macrosomia in diabetic pregnancy is associated with PGM1 phenotype unlikely to be of general significance.