The prevalence of twin pregnancies has growth rapidly in the past several decades, driven largely by increased use of assisted reproductive technologies and older maternal age. However, epidemiologic and clinical evidence focused specifically on twin pregnancies and children remains scant. Early childhood weight trajectory has been studied extensively among singleton children, with results largely supporting an association with numerous downstream health outcomes including overweight and obesity in childhood and adulthood. Minimal evidence in this topic exists for twin children. In this dissertation, I explore normal early childhood weight trajectory and its association with gestational weight gain and with childhood body mass index in twins compared to singletons.