A profile of the soon-to-be Queen Letizia of Spain
For those who wonder about such things, Spain's succession remains male primogeniture. This means sons before daughters, brothers before sisters. Infanta Leonor is the elder of two daughters. When her father becomes the King of Spain, she will be the heiress presumptive, but she will be styled as the Princess of Asturias. She would lose the title only if her mother gives birth to a son, which seems unlikely.
There were discussions by a previous Spanish government to change the constitution to allow for gender equality, but the legislation was never introduced. The change would have required two consecutive Parliaments to pass the law (as well as one other step.)
This has not happened.
Infanta Leonor is the first Princess of Asturias since 1904. Infanta Mercedes of Spain (1880-1904), was the heiress presumptive for her entire life. Alfonso XII's eldest surviving sister, Infanta Isabel (1851-1931) became the Princess of the Asturias for the second when her brother succeeded to the throne. She held the position as heiress presumptive until the birth of Infanta Mercedes, and had previously held the position as heiress presumptive from her birth until 1874, when her brother, Alfonso XII succeeded to the throne.
Maria de last Mercedes was born September 11, 1880, the first child of King Alfonso XII and his second wife, Archduchess Maria Cristina of Austria.
King Alfonso XII died on November 25, 1885, leaving behind two young daughters and a wife, who was pregnant with their child.
Mercedes -- and Spain -- had to wait until Maria Cristina gave birth. If the third child turned out to be another girl, Mercedes would have succeeded her father as Queen Mercedes. The new baby was a boy, King Alfonso XIII from birth.
Mercedes remained the Princess of Asturias until her own death on May 17, 1904, a day after giving birth to a third child, Infanta Isabel Alfonsa.
Three weeks before she was due to give birth, Mercedes suffered from an attack of appendicitis, which was misdiagnosed as cramps. She developed peritonitis, which lead to a premature labor and delivery. The doctors paid more attention to the newborn, and did not notice Mercedes was dying.
Mercedes married her second cousin, Prince Carlo of Bourbon-Two Sicilies. Following her death, her eldest son, Infante Alfonso, became heir presumptive to the Spanish throne. He was not styled as Prince of Asturias.
King Alfonso XIII married Princess Victoria Eugenie of Battenberg on May 31, 1906. A year later, on May 10, 1907, Queen Ena gave birth to her first child, a son, Alfonso, the new Prince of Asturias.
At the time of his first marriage, in June 1933, Alfonso renounced his royal titles. Although his brother, Juan, became the Prince of Asturias, after Alfonso and Jaime's renouncements, he was never officially styled by the title.
It was in January 1977, when Felipe was named as the Prince of Asturias.