To evaluate the association between patterns of gestational weight gain (GWG) and allergic diseases in offspring. Prospective cohort study. Prenatal clinics in Wuhan, China. A cohort of 2546 mother and offspring pairs were enrolled before 16 weeks of gestation and followed up to 24 months postpartum. Maternal body weights were measured regularly during pregnancy, and their GWG patterns were estimated using the growth mixture model. Robust Poisson models were used to evaluate relative risk (RR) and 95%CI after multivariable adjustment. Offspring atopic allergy and allergic contact dermatitis were defined according to a physician's diagnosis reported by the mother, and food allergy was reported by the mother. Three GWG patterns were identified: 18.1% (461) of the women were described as pattern1, characterised by rapid GWG earlier in pregnancy; 56.6% (1442) of the women were described as pattern2, with steady GWG throughout pregnancy; and 25.3% (643) of the women was described as pattern3, with rapid GWG later in pregnancy. By the age of 24 months, 360 (14.1%), 109 (4.3%) and 757 (29.7%) offspring had atopic allergy, allergic contact dermatitis or food allergy, respectively. Compared with women in GWG pattern2, the RRs (95%CIs) among women in pattern1 were 0.74 (0.55-0.99) for atopic allergy, 0.64 (0.36-1.15) for allergic contact dermatitis and 0.95 (0.81-1.12) for food allergy. Maternal GWG pattern characterised by rapid GWG earlier in pregnancy was associated with a lower risk of atopic allergy in offspring.