This study reviews existing evidence for modifiable childhood obesity risk factors that are present from conception to age 2. This period, described as the first 1,000 days, is a critical period for development of childhood obesity and its adverse consequences. The results of this study are based on 282 studies published between January 1, 1980 and December 12, 2014. Several risk factors during the first 1,000 days were consistently associated with later childhood obesity, including higher maternal pre-pregnancy BMI, maternal excess gestational weight gain, prenatal tobacco exposure, high infant birth weight, and accelerated infant weight gain. Other factors had inconsistent results or were only supported by a small number of studies. The author concludes that targeting healthy pre-conceptual weight and gestational weight gain, tobacco avoidance, and healthy infant weight gain with adherence to current infancy nutrition and sleep recommendations shows promise for childhood obesity prevention.