Objective To assess the association between gestational age classification at birth and the risk of neurodevelopmental impairments at age 3 years.

Design Cohort study using the Japan Environment and Children’s Study database.

Patients A total of 86 138 singleton children born without physical abnormalities at 32–41 weeks of gestation enrolled between January 2011 and March 2014.

Main outcome measures Neurodevelopmental impairment, evaluated using the Ages and Stages Questionnaire (third edition).

Methods Logistic regression analysis was used to evaluate the risk of neurodevelopmental impairment in moderate preterm, late preterm and early term children compared with term children after adjusting for socioeconomic and perinatal factors.

Results The respective adjusted ORs (95% CIs) of incidence of scores below the cut-off value (<−2.0 SD) at age 3 years for moderate preterm, late preterm and early term births, compared with full-term births, were as follows: communication, 2.40 (1.54 to 3.73), 1.43 (1.19 to 1.72) and 1.11 (1.01 to 1.21); gross motor, 2.55 (1.69 to 3.85), 1.62 (1.36 to 1.93) and 1.20 (1.10 to 1.30); fine motor, 1.93 (1.34 to 2.78), 1.55 (1.35 to 1.77) and 1.08 (1.01 to 1.15); problem solving, 1.80 (1.22 to 2.68), 1.36 (1.19 to 1.56) and 1.07 (1.00 to 1.14) and personal-social, 2.09 (1.29 to 3.40), 1.32 (1.07 to 1.63) and 1.00 (0.91 to 1.11).

Conclusion Moderate preterm, late preterm and early term births were associated with developmental impairment at age 3 years compared with full-term births, with increasing prematurity. Careful follow-up of non-full-term children by paediatricians and other healthcare providers is necessary for early detection of neurodevelopmental impairment and implementation of available intervention.

The authors do not have permission to share data. Data are unsuitable for public deposition due to ethical restrictions and the legal framework of Japan. It is prohibited by the Act on the Protection of Personal Information (Act No. 57 of 30 May 2003, amendment on 9 September 2015) to publicly deposit the data containing personal information. Ethical Guidelines for Medical and Health Research Involving Human Subjects enforced by the Japan Ministry of Education, Culture, Sports, Science and Technology and the Ministry of Health, Labour and Welfare also restricts the open sharing of the epidemiological data. All inquiries about access to data should be sent to: jecs-en@nies.go.jp. The person responsible for handling enquiries sent to this email address is Dr Shoji F Nakayama, JECS Programme Office, National Institute for Environmental Studies.