The wait goes on for the royal baby. While Prince George is the heir, his younger brother or sister will be 'the spare', ready to step into his place if anything should happen.
Being the spare with no defined role can be an unenviable position. Nonetheless, the history of the English monarchy is full of examples of the Spare becoming monarch.
In the Anglo-Saxon period, it was common for an adult brother to succeed a king rather than infant sons. In this manner, King Alfred succeeded his elder brother Aethelred, while the sons of Edward the Elder: Athelstan, Edgar and Edmund I succeeded in turn. Edmund I's two son, Eadwyg and Edgar also became king in turn (although Edgar was an active participant in an attempt to depose his brother). Edward the Martyr was murdered to make way for his brother, Aethelred II.
In the post-Conquest period it became established that sons succeeded their father in preference to their uncles. In spite of this, the spare often succeeded. William the Conqueror chose his second son, William Rufus, as his heir in England over his eldest son, Robert. Henry I, the youngest of the brothers also became king.
King John was also a brother, but he succeeded Richard I. Richard had also not been their parents' eldest son. This was the short-lived William, while a second son, Henry, died as an adult before becoming king.
Edward I had several sons, including one named Alphonso, but he was eventually succeeded by the youngest son of his first marriage. Richard II, was also a younger brother, becoming heir when his brother died at the age of five. A century later, Richard III, who was his parents' youngest child, very famously became king.
Henry VIII only became heir after the death of his elder brother, Prince Arthur, while Elizabeth I was also a younger child. Charles I became prince of Wales when his very promising adult brother, Prince Henry tragically died. James II and his daughters Queens Mary and Anne were also younger siblings. William IV succeeded his elder brother, George IV, while the current queen's father, George VI, only became king when his elder brother abdicated.
As you can see, the spare is often called upon, by death, sibling childlessness or abdication to step into the heir's place, so the baby born today or later this week might just be very significant indeed. You can read more about the history of the English monarchy in my books, England's Queens: From Boudica to Elizabeth of York and England's Queens: From Catherine of Aragon to Elizabeth II.
By the way, let me know if I've missed anyone above!