AbstractAimsTo determine the fetal and maternal outcomes in pregnant women with Glucokinase‐Maturity onset diabetes of the young (GCK‐MODY).MethodsWe studied the obstetric and perinatal outcomes in 99 pregnancies of 34 women with GCK‐MODY. The mutation status of the offspring was known in 29 and presumed in 33. Clinical outcomes were determined and compared between affected (n = 39) and unaffected (n = 23) offspring.Results59% of pregnancies were treated with diet alone and 41% received insulin. Birthweight, percentage of large for gestational age (LGA) and caesarean section (CS) in GCK‐unaffected offspring was significantly higher than in GCK‐affected offspring (4.0 ± 0.7 vs. 3.4 ± 0.4 kg, p = 0.001), 15 (65%) vs. 5(13%) (p = 0.00006) and 17 (74%) vs. 11 (28%) (p = 0.001), respectively. We observed an earlier gestational age at delivery on insulin in unaffected offspring (38.3 ± 1.0 vs. 39.5 ± 1.5 weeks, p = 0.03) with no significant change in LGA (9 (82%) vs. 6 (50%); p = 0.12), and a higher rate of CS (8 [73%] vs. 3 [11%]; p < 0.001), and no change in small for gestational age (0 [0%] vs. 4 [14%]; p = 0.30) in affected offspring.ConclusionInsulin therapy in unaffected offspring did not reduce LGA and was associated with earlier gestational age at delivery. Insulin treatment in GCK‐affected offspring was associated with an increased incidence of CS, but did not adversely affect fetal outcome. Fetal genotype determines birthweight rather than treatment. Pre‐pregnancy diagnosis of GCK‐MODY, use of continuous glucose monitoring and non‐invasive fetal genotyping may enable further investigation of targeted therapy in this condition.