Aims: To examine the relationship between length of the preceding birth interval and neurodevelopmental outcomes in childhood in a birth cohort study from a high-income country (ALSPAC ) and in two birth cohorts from a middle income country (the 1993 and 2004 Pelotas birth cohort studies, from Brazil).

These studies were chosen to reflect populations with different levels of wealth and of socioeconomic inequalities, as well as their similarities in variable definitions and the availability of comparable questionnaires and follow-ups done at similar ages.

Hypotheses: Previous studies have shown the association between short inter-pregnancy intervals (especially those shorter than 12 months) and several maternal and perinatal adverse outcomes. Recently short inter-pregnancy intervals have also been associated with an increased risk of developing autism and schizophrenia. It is hypothesised that the risk of adverse neurodevelopment outcomes may be high among children that are conceived following a short inter-pregnancy internal, when maternal folate stores are still being replenished. The period immediately following the birth of a preceding pregnancy may be a particular period of risk where depleted maternal nutrient reserves could affect neurogenesis during early fetal development. In order to examine causality of the association being tested, we will also examined the association between post-birth inter-pregnancy interval (time between the index birth and conception of the subsequent live birth), where the hypothesis regarding folate/micronutrien depletion would not apply.