Dying of an illness, Melody Pond initiated her first regeneration in New York City in 1970. (TV: Day of the Moon) There, she became a toddler who came to be known as Mels Zucker, a childhood friend of Melody's parents, Amy Pond and Rory Williams. Mels herself would regenerate into River Song who expended her remaining regeneration energy to save the life of the Eleventh Doctor. (TV: Let's Kill Hitler, WC: The Raggedy Doctor by Amelia Pond)
It’s easy really. See?