<b><i>Background:</i></b> Early infant weight development influences metabolic regulation later in life. For the prevention of obesity and metabolic diseases, it is important to understand the underlying mechanisms in detail. <b><i>Objectives:</i></b> This study aims to examine the effects of maternal anthropometric, sociodemographic, and lifestyle factors on maternal and cord blood leptin levels at birth and on the development of body mass index (BMI) standard deviation scores (SDS) in offspring up to 1 year of age. <b><i>Methods:</i></b> Seventy-six mother-child pairs were enrolled in this follow-up analysis in a cross-sectional design. Standardized questionnaires were used to collect information regarding maternal anthropometrics, lifestyle habits, and sociodemographic conditions, and newborn weight, or, rather, BMI-SDS, development during the first year of life. <b><i>Results:</i></b> Cord blood leptin (β = –0.222, <i>p</i> = 0.074), maternal leptin (β = 0.414, <i>p</i> = 0.001), and female sex of the offspring (β = 0.385, <i>p</i> = 0.003) explained 29.0% of the variance in BMI-SDS changes in the first year of life. Cord blood leptin was influenced by newborn sex (male; β = –0.220, <i>p</i> = 0.025) and maternal moderate-intensity physical activity in the third trimester (β = 0.265, <i>p</i> = 0.007, corr. <i>R</i>² = 9.2%); maternal leptin was influenced by maternal prepregnancy BMI (β = 0.602, <i>p</i> &#x3c; 0.001) and weight gain during pregnancy (β = 0.247, <i>p</i> = 0.004, corr. <i>R</i>² = 35.5%). <b><i>Conclusions:</i></b> Higher maternal and lower cord blood leptin levels are associated with a higher BMI-SDS increase during the first year of life. Maternal leptin is influenced by maternal BMI and weight gain during pregnancy, and cord blood leptin is influenced by maternal physical activity; therefore, it can be suggested that an active and healthy maternal lifestyle may play a pivotal and beneficial role in the offspring’s weight development.