In 1543 King Henry VIII signed the Treaty of Greenwich, which included the betrothal of the future King Edward VI of England to his cousin, Mary the future Queen of Scots. Such a marriage could potentially have united the two crowns through the birth of one son and heir. Ultimately it failed as Edward died and Mary married the French Dauphin. But it wasn’t the first time such a marriage had been mooted, and it had also ended with an early death. 

Edward I and his wife Eleanor of Castile are known for having a large brood of children. Eleanor is believed to have given birth to sixteen children. However many of these children died young, and it took a long time before they had a surviving male heir. Edward of Carnarvon was the youngest of the sixteen and the fourth boy born to the couple. Had they survived, any of his brothers would have been King of England instead of him. 

A Danish princess by birth and a Queen of France by marriage, Ingeborg of Denmark tends to be forgotten about. The great Royal marriage battle between King and Queen that most people remember, is the one between King Henry VIII of England and his first wife Catherine of Aragon. But Ingeborg's own battle was not only far more protracted, it was ultimately successful. 

Born on 5 August 1103 William Adelin would have been the first Anglo-Norman King, reflecting the changes in English society since the Conquest. While his father was King Henry I of England, the youngest son of William the Conqueror, his mother was Matilda (formerly Edith) of Scotland. Her mother had been a granddaughter of the old Anglo-Saxon kings, and had married King Malcolm III of Scotland in 1070. William Adelin might even have had a claim on the Scottish throne if he had survived. 

Although her early life can classify her as an “Unlucky Princess”, in many ways Margaret of Austria’s life was better than other such royal women. Betrothed and married several times, she was not only eventually allowed to manage her own destiny, but she became ruler of the Netherlands as Regent for her nephew. Through this she joined the ranks of other strong women who managed the region on behalf of their menfolk. 

Unless he thought about changing his name, as later Kings did on accession, then Britain could have had it's first King Fred in the 18th century. On his birth in 1707 the idea that the Electors of Hanover would become Kings of Great Britain was becoming more obvious. Queen Anne had no surviving children, and Frederick's great-grandmother Sophia was Anne's nearest Protestant relative. 

You might have noticed so far that although the unlucky princesses have generally been unhappy in their marriages, none of them have specifically been murdered on the orders of their unpleasant spouse.

Step forward Blanche of Bourbon. 

One of the most famous losses to the English throne, Edward the Black Prince was extremely popular for most of his life, but ended it a sick, bitter man. 

Caroline Matilda was born into a court in mourning. Her father Frederick, Prince of Wales, had died four months earlier leaving his wife Princess Augusta a widow with nine children. The family had long been estranged from Frederick's father, King George II of England, but the Princes' death led to a cooling of tensions. Although George didn't like Augusta, and didn't like her reluctance to take part in court activities, she was generally left to raise her children as she saw fit. 

At his birth Eustace of Boulogne couldn't have hoped to become anything more than a nobleman, like his father and grandfather. He was due to inherit his mother's county of Boulogne, Stephen himself was a younger brother who had married the heiress Matilda of Boulogne, and had taken title Count of Boulogne through her. As the eldest son Eustace could look forward to becoming Count Eustace of Boulogne in the future. But in infancy he gained a new title and new future – Prince Eustace, heir to the English throne.