Background: Successes in improving child survival in low- and middle-income countries mean that the time is right to expand the child health agenda to encompass the ambition that all children are able to thrive. This expansion will have profound and positive implications for families, communities and societies because supporting children to thrive in the earliest years allows them to reach their potential through later child- and adulthood. Unfortunately, current estimates are that at least 43% of under 5 year olds in low and middle-income countries are at high-risk of suboptimal development because of the many obstacles they face, including poverty, family stress, malnutrition, exposure to violence amongst others. This PhD examined the associations between number and types of these adversities and stress through pregnancy and the first year of life, and growth and development at 18 months of age in SPRING-ELS (Early Life Stress). This was a sub-study of the community-based SPRING trial which developed a home visits intervention programme, delivered at-scale across a total population of 200,000 in rural India where the burden of adversity is great, and indicators of child wellbeing including early childhood stunting (low height-for-age) are shocking. Methods: SPRING-ELS was initiated with formative work to understand mother’s perceptions of childhood stress and adversity. These findings were combined with literature reviews and expert consultation to select 22 childhood adversities covering pregnancy and the first year of life in four domains (socioeconomic, carer-child relationship, maternal stress and child-related). Data on these adversities were collected at 12-months of age, except for socioeconomic status and parental education which had already been collected when children were identified by the trial surveillance system. Hair was sampled at 12-months to analyse chronic stress using cortisol, and six saliva samples were taken over two days to assess children’s diurnal cortisol rhythm. Growth and development outcomes were assessed when children were 18 months old; these were height-for-age, weight-for-age and the motor, cognitive and language domains of Bayley Scales of Infant Development 3rd edition. The relationships between these measures of adversity, cortisol stress measures, and early childhood growth and development were assessed using multi-level mixedeffects linear regression modelling. Results Adversity, growth and development data were collected on 1273 children; saliva cortisol results were available for 752 of these children and hair cortisol for 712. Most children (90.6%) faced at least 1 adversity and nearly 50% faced four or more of these potential impediments to wellbeing. Each additional adversity was associated with worsening weight-for-age (-0.09 (95%CI -0.11, -0.06)) and height-for-age (-0.12 (95% CI -0.14, -0.09)) z-scores. There were similar negative associations for child development as follows: -1.1 points (95% CI -1.3, -0.9) for motor, -0.8 points (95% CI -1.0, -0.6) for cognitive; and -1.4 points (95% CI -1.9, -1.1) for the language scale. Comparable results were found for each of the four adversity domains. Number of adversities was also strongly linked to concentration of cortisol in the hair (increase of 6.1% (95% CI 2.8, 9.4) for each additional adversity) but not with diurnal cortisol rhythm in saliva. Finally, hair cortisol itself strongly associated with poorer growth and development in all domains; this was predominantly independent of a child’s exposure to adversity. Conclusions: This PhD presents amongst the first evidence from a low/middle-income country showing the relationship between numbers of childhood adversities in multiple domains and growth and development at only 18 months of age, a crucial time for optimal brain development and a key predictor of future health and wellbeing. It is a reminder that children are never too young to absorb what is happening around them, and that the damaging effects of adversity on developing biological systems begins from the start. The findings reinforce the need for a multidimensional approach to tackling the burden of suboptimal growth and development, and that this approach should include a focus on early life adversity and stress because of the profound implications of these for child wellbeing. The World Health Organization and UNICEF have pioneered such a multidimensional approach at the global level through the Nurturing Care Framework and urgent translation of this into country-level action is now needed to support all children – no matter where they live – to thrive.