The Horizon2020 LifeCycle Project is a cross-cohort collaboration which brings together data from multiple birth cohorts from across Europe and Australia to facilitate studies on the influence of early-life exposures on later health outcomes. A major product of this collaboration has been the establishment of a FAIR (findable, accessible, interoperable and reusable) data resource known as the EU Child Cohort Network. Here we focus on the EU Child Cohort Network’s core variables. These are a set of basic variables, derivable by the majority of participating cohorts and frequently used as covariates or exposures in lifecourse research. First, we describe the process by which the list of core variables was established. Second, we explain the protocol according to which these variables were harmonised in order to make them interoperable. Third, we describe the catalogue developed to ensure that the network’s data are findable and reusable. Finally, we describe the core data, including the proportion of variables harmonised by each cohort and the number of children for whom harmonised core data are available. EU Child Cohort Network data will be analysed using a federated analysis platform, removing the need to physically transfer data and thus making the data more accessible to researchers. The network will add value to participating cohorts by increasing statistical power and exposure heterogeneity, as well as facilitating cross-cohort comparisons, cross-validation and replication. Our aim is to motivate other cohorts to join the network and encourage the use of the EU Child Cohort Network by the wider research community.

IMPORTANCEPeople conceived using assisted reproductive technology (ART) make up an increasing proportion of the world's population. OBJECTIVE To investigate the association of ART conception with offspring growth and adiposity from infancy to early adulthood in a large multicohort study. DESIGN, SETTING, AND PARTICIPANTS This cohort study used a prespecified coordinated analysis across 26 European, Asia-Pacific, and North American population-based cohort studies that included people born between 1984 and 2018, with mean ages at assessment of growth and adiposity outcomes from 0.6 months to 27.4 years. Data were analyzed between November 2019 and February 2022. EXPOSURES Conception by ART (mostly in vitro fertilization, intracytoplasmic sperm injection, and embryo transfer) vs natural conception (NC; without any medically assisted reproduction). MAIN OUTCOMES AND MEASURESThe main outcomes were length / height, weight, and body mass index (BMI; calculated as weight in kilograms divided by height in meters squared). Each cohort was analyzed separately with adjustment for maternal BMI, age, smoking, education, parity, and ethnicity and offspring sex and age. Results were combined in random effects meta-analysis for 13 age groups. RESULTSUp to 158 066 offspring (4329 conceived by ART) were included in each age-group metaanalysis, with between 47.6% to 60.6% females in each cohort. Compared with offspring who were NC, offspring conceived via ART were shorter, lighter, and thinner from infancy to early adolescence, with differences largest at the youngest ages and attenuating with older child age. For example, adjusted mean differences in offspring weight were −0.27 (95% CI, −0.39 to −0.16) SD units at age younger than 3 months, −0.16 (95% CI, −0.22 to −0.09) SD units at age 17 to 23 months, −0.07 (95% CI, −0.10 to −0.04) SD units at age 6 to 9 years, and −0.02 (95% CI, −0.15 to 0.12) SD units at age 14 to 17 years. Smaller offspring size was limited to individuals conceived by fresh but not frozen embryo transfer compared with those who were NC (eg, difference in weight at age 4 to 5 years was −0.14 [95% CI, −0.20 to −0.07] SD units for fresh embryo transfer vs NC and 0.00 [95% CI, −0.15 to 0.15] (continued)

Background Sleep is important for healthy functioning in children. Numerous genetic and environmental factors, from conception onwards, may influence this phenotype. Epigenetic mechanisms such as DNA methylation have been proposed to underlie variation in sleep or may be an early-life marker of sleep disturbances. We examined if DNA methylation at birth or in school age is associated with parent-reported and actigraphy-estimated sleep outcomes in children. Methods We meta-analysed epigenome-wide association study results. DNA methylation was measured from cord blood at birth in 11 cohorts and from peripheral blood in children (4–13 years) in 8 cohorts. Outcomes included parent-reported sleep duration, sleep initiation and fragmentation problems, and actigraphy-estimated sleep duration, sleep onset latency and wake-after-sleep-onset duration. Results We found no associations between DNA methylation at birth and parent-reported sleep duration (n = 3658), initiation problems (n = 2504), or fragmentation (n = 1681) (p values above cut-off 4.0 × 10–8). Lower methylation at cg24815001 and cg02753354 at birth was associated with longer actigraphy-estimated sleep duration (p = 3.31 × 10–8, n = 577) and sleep onset latency (p = 8.8 × 10–9, n = 580), respectively. DNA methylation in childhood was not cross-sectionally associated with any sleep outcomes (n = 716–2539). Conclusion DNA methylation, at birth or in childhood, was not associated with parent-reported sleep. Associations observed with objectively measured sleep outcomes could be studied further if additional data sets become available.

There is growing evidence that insufficient sleep has negative effects on the mental health of children. The aim of this study is to examine the associations between device-measured sleep duration and internalizing and externalizing problems in 8-year-old children. The study is a secondary analysis of data from the Childhood Obesity Project conducted in five European countries. Nocturnal sleep duration was measured with the SenseWear™ Armband 2. Parents rated their child’s internalizing and externalizing problems on the Child Behaviour Checklist. Behaviour scores were dichotomized at the 90th percentile based on sex- and country-specific z-scores. Logistic regression models were applied to test the associations between sleep duration and behaviour. Data were available for 406 8-year-old children. The average sleep duration was 9.25 h per night (SD: 0.67) with 1464 nights measured in total. The sleep duration recommendation of the American Academy of Sleep Medicine for school-aged children (9–12 h) was met by 66.7% of children. One hour of additional sleep per night significantly reduced the risk of having internalizing problems (adjusted OR = 0.51; 95% CI 0.29–0.91). Children who adhered to the sleep duration recommendation had a lower risk for internalizing problems (adjusted OR = 0.45; 95% CI 0.21–0.99). Sleep duration and externalizing problems showed no significant association. Longer sleep duration was associated with a reduced risk of having internalizing problems but not externalizing problems. Results highlight that it is important to ensure adequate sleep duration throughout primary-school years for the optimal emotional health of children. Trial registration number: NCT00338689. Registered: June 19, 2006.

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