Dietary glycaemic index and glycaemic load (GL) have been related to obesity and other health outcomes. The objective of the present study was to examine the associations between maternal dietary GL and gestational weight gain, birth weight, the risk of giving birth to a child large-for-gestational age (LGA) or small-for-gestational age and postpartum weight retention (PPWR). Data were derived from the Danish National Birth Cohort (1996–2002), including data on gestational and lifestyle factors in pregnancy and 18 months postpartum. Dietary data were collected using a validated FFQ. Information on birth outcome was obtained through registers. A total of 47 003 women were included. The associations between the GL and birth outcome, gestational weight gain, assessed between weeks 12 and 30 of gestation, and PPWR were analysed by linear and logistic regression. Birth weight increased by 36 g from the lowest to highest GL quintile (95 % CI 19, 53 g), and an increased risk of LGA of 14 % was detected in the highest GL quintile compared with the lowest GL quintile. Among normal-weight and overweight women, higher gestational weight gain rates were detected in the highest GL quintile (26 g/week (95 % CI 19, 34) and 30 g/week (95 % CI 13, 46), respectively). The association between the GL and PPWR was most pronounced among pre-pregnant obese women, with an increase in weight retention of 1·3 (95 % CI 0·2, 2·8) kg from the lowest to highest GL quintile. The GL may play a role for excessive gestational weight gain and PPWR, which may be more pronounced among overweight and obese women.