Objective Excessive gestational weight gain (GWG) is an important contributing factor to the obesity epidemic in women and is associated with pregnancy complications. We investigated the relationship between GWG and caesarean delivery in labour, large for gestational age (LGA), small for gestational age (SGA) infants and pregnancy-induced hypertension by maternal pre-pregnancy body mass index (BMI) in a contemporary nulliparous cohort. Study design Using 2009 Institute of Medicine guidelines, participants in the SCOPE study (from Cork, Ireland, Auckland, New Zealand and Adelaide, Australia) were classified into GWG categories (low, normal and high) according to pre-pregnancy BMI. Maternal characteristics and pregnancy outcomes were compared between weight gain categories. SGA and LGA were defined as 90th customised birthweight centile. Multivariable analysis adjusted for confounding factors that impact on GWG including BMI. Results Of 1950 participants, 17.2% (n = 335) achieved the recommended GWG, 8.6% (n = 167) had low and 74.3% (n = 1448) had high GWG. Women with high GWG had increased rates of LGA infants [adjusted OR 4.45 (95% CI 2.49–7.99)] and caesarean delivery in labour [aOR 1.46 (1.03–2.07)]. SGA was increased in women with low GWG [aOR 1.79 (1.06–3.00)]. Conclusion Three quarters of participants had high GWG, which was associated with an independent risk of LGA infants and caesarean in labour. Low GWG was associated with SGA infants. These adverse outcomes are potentially modifiable by achievement of normal GWG, which should be an important focus of antenatal care.