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Background Unreasonable use of antibiotics and probiotics can alter the gut ecology, leading to antibiotic resistance and suboptimal health outcomes during early life. Our study aims are to clarify the association among antibiotic and probiotic exposure in early life, the microecology of the gut microbiota, and the development of antibiotic resistance; to investigate the long-term impact of antibiotics and probiotics on the health outcomes of infants and young children; and to provide a theoretical basis for the rational use of antibiotics and probiotics from a life course perspective. Methods The study is a prospective, longitudinal birth cohort study conducted in Shaanxi Province, China from 2018 to 2024. A total of 3,000 eligible mother–child pairs will be enrolled from rural, suburban, and urban areas. The recruitment of the participants begins at pregnancy, and the newborns will be followed up for 2 years at successive timepoints: within 3 days after birth, 42 days after birth, and at 3, 6, 12, 18, and 24 months of age. Sociodemographic data, environmental exposures, dietary patterns, psychological conditions, and medical and drug histories are collected. Cognitive and behavioural development among infants and young children and questionnaires on antibiotic knowledge and behaviour among caregivers will be collected at 12 and 24 months of age. The faecal samples are collected and analysed by 16S rRNA high-throughput sequencing and quantitative PCR (qPCR) for antibiotic resistance genes. Discussion The findings will inform antibiotic and probiotic use for pregnant women and infants and contribute to establishing rational use strategies of antibiotics and probiotics for paediatricians, health practitioners, and drug administration policy-makers. Trial registration The study was registered on the Chinese Clinical Trial Registry (ChiCTR) platform, http://www.chictr.org.cn (Record ID: ChiCTR2100047531, June 20, 2021).

Purpose The associations of early-life head circumference (HC) with child cognition and emotional and behavioral health among generally healthy population remain unclear. We aimed to examine the associations of early-life HC trajectories with adolescent neurodevelopmental and mental health and to identify HC growth-sensitive period. MethodsWe conducted a prospective, community-based birth cohort in rural western China, and 745 adolescents aged 10-14 years were followed between June to December 2016. We repeatedly assessed child HC for eight times during the first two years of life, and adolescent cognitive and emotional and behavioral outcomes using Wechsler Intelligence Scale for Children and Youth Self-Report, respectively. We applied group-based trajectory modeling to identify HC trajectories and used conditional growth to derive sensitive periods. ResultsWe identified five distinct HC trajectories characterized as Start below average-then decrease (7.8% of the sample), Start below average-then increase (6.8%), Start average-then decrease (33.4%), Consistently average (37.9%), and Consistently above average (14.1%). Infants in the trajectory of consistently above average had higher cognitive scores at early adolescence as compared to those from suboptimal trajectories, with adjusted mean differences ranging from -2.84 to -8.99 points. The conditional gains show that the sensitive HC growth period appeared to be between 0-6 months. While we found null associations between HC measures and adolescent emotional and behavioral health.ConclusionsRoutine HC measurements especially during the first 6 months were recommended for monitoring child cognition but for mental health. Integrated prenatal and postnatal interventions should be pursued to promote child development.