Originally, it was actually the other way around. For much of the 19th century, most infants and toddlers wore white dresses. Dresses facilitated diaper-changing, and white cotton could easily be cleaned with bleach. But around 1900, childcare experts began to push for a greater distinction between little girls and boys, amid fears that boys were growing up “weaker” and “lazier” than their fathers had. Many U.S. publications and stores responded in part by recommending pink clothing for boys and blue clothing for girls, although some also recommended the opposite color scheme. According to Dressmaker magazine, “Blue is reserved for girls as it is considered paler, and the more dainty of the two colors, and pink is thought to be stronger (akin to red).”
But around World War II, everything changed. Soon pink was heavily marketed as the preferred color for girls, and blue for boys. It’s not entirely clear what led to the switch, and the colors chosen were somewhat arbitrary — the focus was primarily on creating clothes specific for each child in an attempt to curb hand-me-downs, and thus sell more product. Once the 1950s began, hospitals wrapped newborns in pink or blue blankets, based on their sex (today’s standard blankets contain pink and blue stripes).