Before Saint Louis—King of France—was born, his mother Blanche de Castile (1) had three boys, two of whom died in infancy, while the third, Philip, died at the age of 8 or 9 when Louis was 3 or 4 years old. The royal parents, seeing their male progeny extinguished from such a young age, were greatly concerned to give a future heir to the Kingdom, especially because it was feared that Louis, born in April 1214, would not survive either.
The queen was very devout, so she confided her torment to a monk called Dominic of Guzman (1170-1221), the founder of the Dominican Order and future Saint Dominic, during a visit he paid her. Dominic advised her to say her Rosary every day to obtain from Heaven this much-desired grace. From that day, the queen had more than ever recourse to the Blessed Virgin: she recited her Rosary every day and had many rosaries distributed to the whole Court as well as in several cities of the kingdom, hoping that God would give her a full blessing.
Her prayer was answered: Saint Louis was the great king who would bring glory to France (he would have seven younger brothers and sisters).
(1) Saint Blanche de Castille (1188-1252), was the wife of Louis VIII, King of France and the mother of Saint Louis (Louis IX), King of France.
Jean-Jacques Riou, 30 minutes pour devenir saintes et saints avec le chapelet au quotidien (excerpts, translated), Saint-Léger Éditions. Preface by Bishop Dominique Rey of Fréjus, France.