Background: Reproductive health problems, such as shifts in puberty timing, infertility, and recurrent pregnancy loss, are increasingly recognised as major public health concerns, and early life behaviours are emerging as important determinants of these outcomes. Therefore, this systematic review summarised current evidence on the effects of early-life nutrition, physical activity, and sleep on reproductive health outcomes later in life, and assessed methodological limitations in these existing studies.

Methods: Six major databases were searched through July 2025, and eligible studies included longitudinal and case-control designs assessing exposures (diet, physical activity, and sleep) from the prenatal period to middle adolescence and reproductive outcomes (puberty timing, infertility, and pregnancy loss) from puberty onward. Two reviewers independently screened the studies, evaluated the quality of the included articles, and extracted the data. The use of causal inference methods was also evaluated. Findings were narratively synthesised.

Results: Fifty-nine studies met the inclusion criteria (55 longitudinal and 4 case-control). Most studies examined puberty timing, while only four assessed fertility and one examined pregnancy loss. Adverse early-life dietary exposures, including high intake of animal protein, total fat, polyunsaturated fatty acids, red and processed meats, and sweetened or caffeinated beverages, were consistently associated with earlier puberty. Nine of eleven studies reported that inadequate physical activity was associated with earlier puberty. Sleep findings were limited and mixed. Evidence for infertility and pregnancy loss was sparse and inconclusive. Only six studies used directed acyclic graphs, and none applied causal effect estimation methods.

Conclusion: Early life diet, physical activity, and sleep appear to influence puberty timing, but evidence for infertility and pregnancy loss remains limited. More research is needed to understand the effects of early life behaviours on infertility and pregnancy loss, and more rigorous study designs, including the application of causal inference approaches, are required.