Date and order of birth
There is some disagreement concerning the order of birth of Sancho III's sons, and of Ferdinand's place among them. He was certainly a younger son, and he was probably born later than 1011, by which date his parents are known to have married. Most, and the most reliable, charters name Sancho's sons in the order Ramiro, García, Gonzalo, then Ferdinand. Three documents from the Cathedral of Pamplona list them in this way, as well as four from the monastery of San Juan de la Peña. One charter from Pamplona, dated 29 September 1023, is witnessed by Sancho's mother,
Jimena Fernández, his wife Muniadona, her children, listed García, Ferdinand then Gonzalo, and their brother, the illegitimate Ramiro.
In five documents of the monastery of San Salvador de Leire, Ferdinand is listed after Gonzalo. Two of these are dated to 17 April 1014. If authentic, they place Ferdinand's birth before that date. Three further documents from Leire are among the only ones to place Ferdinand second among the legitimate sons, but they suffer from various anachronisms and interpolations. Two preserved diplomas of Santa María la Real de Irache also put Gonzalo ahead of him. On the basis of these documents, Gonzalo Martínez Díez places Ferdinand third of the known legitimate sons of Sancho III (Ramiro being a bastard born before Sancho's marriage to Muniadona), and his birth no earlier than 1015. The Crónica de Alaón renovada, which Martínez Díez dates to 1154, but which other scholars dismiss as a late medieval concoction, lists García, Ferdinand and Gonzalo as Sancho III's sons by Muniadona in that order, but in the same passage mistakenly places Gonzalo's death before his father's.