In the royal palace in Vienna, late in the winter of 1673, an empress lay dying. Margaretha Theresa of Austria was twenty-one years old, pregnant for what would have been her fifth time in six years of marriage, and struggling to breathe. The swelling in her throat that had troubled her since childhood had been joined, that season, by an infection in her chest. She was the daughter of the King of Spain, the wife of the Holy Roman Emperor, and the girl in the center of the world’s most studied painting.

None of that could help her now. A few years earlier, in 1656, the Spanish court painter Diego Velázquez had placed a small princess at the center of a canvas that would become known as *Las Meninas*. The painting hangs today in the Prado Museum in Madrid. The princess, five years old, stands surrounded by her ladies-in-waiting.

Velázquez painted himself on the left, brush in hand, before a vast canvas. In the background, on the rear wall, hangs a small mirror. The two blurred faces reflected in it are the girl’s parents: King Philip IV of Spain and his wife, Queen Mariana of Austria. What the painting cannot show is who those parents were to each other.

Mariana was Philip’s niece. The old king had married his brother’s daughter, and the little girl in the center of the room was their child. At fifteen, Mariana had been wed to her uncle. By the age of twenty-one, she had buried three of her children and died herself.

The one child she left behind would carry the highest coefficient of inbreeding ever calculated for any member of the Habsburg family. That child was the girl in the painting. Her name was Margaret Theresa. For the Habsburgs, a marriage like that of Philip and Mariana was not abnormal.

The family ruled Spain, Austria, and vast stretches of Europe, and they held it all together by marrying one another. Cousin to cousin, uncle to niece, generation after generation, so that the crowns of Europe would never leave the family. Every one of those marriages drew from the same small pool of blood. When geneticists later calculated the inbreeding levels of Philip and Mariana’s children, they found them comparable to what would be expected of an offspring of a brother and sister.

One of those children was Margaret Theresa’s younger brother, the sickly boy who would grow up to become Charles II of Spain, the last Habsburg king of that country. But Margaret Theresa herself survived the severe disabilities that would plague her brother. She was born on July 12, 1651, in the royal palace in Madrid. Contemporary accounts describe her as slender, delicately built, yet lively and intelligent.

Velázquez painted her again and again as she grew—eight times, by one count. In 1659, one of those portraits, the so-called *Girl in the Blue Dress*, was sent as a gift to the imperial court in Vienna. Within a year, Spain had promised her in marriage to the emperor who lived there. That emperor was Leopold I.

He was her mother’s brother, which made him her uncle. His mother had been her father’s sister, which also made him her first cousin. By one modern genetic estimate, Leopold was the most inbred Holy Roman Emperor ever to sit on the throne. The engagement was announced in 1663, when Margaret Theresa was eleven years old.

At that point, Spain made a decision that cast a long shadow over the family for decades. When Margaret Theresa’s older half-sister had married the King of France, she had been forced to renounce her claims to the Spanish throne. Margaret Theresa was allowed to keep hers. All across Europe, this was seen as a political sensation.

It meant that a Habsburg bride in Vienna might one day pass a claim to Spain and its entire empire down to her children. On April 25, 1666, at the age of fourteen, Margaret Theresa married Leopold by proxy in the Alcázar Palace in Madrid, with a stand-in playing the groom’s part. Then came the long journey over the Alps to Austria. In late November, she met Leopold for the first time, at Schottwien, on the road south of Vienna.

In December of that year, now fifteen, she married him in person. Vienna celebrated the wedding for nearly two years. In January 1667, Leopold performed in a massive equestrian ballet in the courtyard of the Hofburg Palace, a production designed around her name, because Margaret in Latin means *pearl*. For her seventeenth birthday, the court staged *Il Pomo d’Oro* (“The Golden Apple”), one of the most extravagant operatic productions of the age.

By most accounts, the marriage itself was happy. The couple shared a love of music, and it was said she called her husband “uncle. ” She remained Spanish to the core, surrounded by Spanish servants, and barely learned German. Then the pregnancies began, and they barely stopped.

Her first son, Ferdinand Wenzel, was born in September 1667. He died in January of the following year, not yet four months old. In January 1669, she gave birth to a daughter named Maria Antonia. That name matters, because Maria Antonia is the person this story is really about.

A year later, in February 1670, Margaret Theresa went into early labor. The boy she delivered, Johann Leopold, died the same day he was born. During that birth, she made a vow. If she survived, she would work to expel the Jews from Vienna.

She did survive. In that same year, Vienna’s Jewish community was expelled. Their synagogue was torn down, and the neighborhood where they had lived was renamed Leopoldstadt, after her husband. It still carries that name today.

It is a grim detail, and it shows how much weight one royal birth carried inside that palace—and who ended up paying the price for it. In February 1672, she gave birth to another daughter. The child lived only two weeks. Count the toll: five years of marriage, four children born, three buried, and by some accounts, two more pregnancies lost before they came to term.

In early 1673, she was pregnant again. She had suffered from a swelling in her throat since childhood. That winter, she caught a chest infection on top of it. The standard German biography of her records that she died of suffocation.

She died in Vienna on March 12, 1673, at twenty-one years old. Other accounts attribute her death to sheer exhaustion after nearly six years of almost constant pregnancy. The exact cause is still debated. Then comes the cruelest sentence in her entire story.

The Swedish envoy at the imperial court, Esaias Pufendorf, wrote in his reports that people in Vienna regarded her death in her twenty-first year as a stroke of luck for the Austrian line. A young empress dies in pregnancy, and the talk in the palace corridors was that the dynasty had been fortunate. Leopold mourned her as the only Margaret of his life. Within months, he was arranging his next marriage.

Her only surviving child was a four-year-old girl living in the Hofburg Palace. That girl inherited everything her mother owned, including the claim to the Spanish throne. Her family tree was even more tangled than her mother’s. Her father was her mother’s uncle.

Her mother was herself the child of an uncle and a niece. Trace the branches upward, and the same people appear in different places. On her father’s side, her grandparents were Emperor Ferdinand III and Maria Anna of Spain. On her mother’s side, that same couple were her great-grandparents.

A normal family tree has eight great-grandparents. Hers had six. By the time she was seven years old, the family had already chosen a husband for her. In 1676, a marriage contract was signed promising her to her mother’s brother, Charles, the King of Spain.

It would have been the third uncle-niece marriage in three generations. But it never took place. Still, for about twenty years, because Charles was childless, the Spanish court treated this girl in Vienna as the heir to the entire Spanish Empire. No one who met her described anything monstrous about her.

The surviving accounts say she was quiet and reserved, shy and delicate. Not a great beauty, but striking in her own way. There is no reliable record of the famous Habsburg jaw, or any of the deformities you might expect from a family tree like hers. Her name was Maria Antonia of Austria.

In 1685, at sixteen years old, she married Maximilian Emmanuel, the Elector of Bavaria, in Vienna. As part of the agreement, she renounced her claim to the Spanish throne, keeping only the Spanish Netherlands, which were promised to her new husband. The marriage turned sour. The standard biography says Maximilian Emmanuel treated her unbearably, and she could not keep his mistresses away from him.

Then her body began to repeat her mother’s story. She gave birth to three sons; the first two did not survive infancy. In 1692, she was pregnant again when her husband left for Brussels. She refused to go with him.

Instead, she returned to her father in Vienna. On October 28, 1692, in the Hofburg Palace, she gave birth to a boy named Joseph Ferdinand. This time, the child lived. But she never regained her strength.

The weeks passed. She sank into a deep melancholy. On Christmas Eve, 1692, she died. She was twenty-three years old.

The exact cause of death was not recorded clearly enough to be determined. The theories you will find online, from childbed fever to depression, are only guesses. Before her death, she made sure of one thing. In her will, she renounced the Spanish crown again.

She excluded her husband entirely. She left everything she owned to her infant son, with only a few keepsakes offered to the Bavarians. She asked to be buried beside her mother in the Imperial Crypt in Vienna. So how inbred was she, really?

In 2013, two geneticists, Francisco Ceballos and Gonzalo Álvarez, reconstructed the Habsburg family tree across more than twenty generations and over four thousand people. They calculated an inbreeding coefficient for each individual. That number is the probability that the two copies of a gene a person inherits are identical because they come from the same ancestor. For a child of unrelated parents, the figure is near zero.

For a child of first cousins, it is about 0. 06. For an uncle and niece, 0. 125.

For a full brother and sister, 0. 25. Maria Antonia’s result came in at 0. 3053.

That is the highest number they found anywhere in the Habsburg line. Higher than her uncle Charles, who scored 0. 254. Higher than what you would expect for the child of a brother and sister.

The reason is that these ratios accumulate. Her parents were uncle and niece, and first cousins at the same time, and every marriage in the generations before them drew from the same tight circle of people. Each marriage pushed the number a little higher. A truth must be stated here, because it matters.

That number does not come from her DNA. No one has ever examined her remains. It is calculated from family trees, and part of the database behind it was built from genealogy websites, including Wikipedia. So 0.

3053 is only an estimate on paper. But it is a peer-reviewed estimate, and no one else in that family ever scored higher. What the research points to is damage you cannot see in a painting. Between 1527 and 1661, nearly three out of every ten children born into the Spanish royal family died before their first birthday, at a time when ordinary Spanish villagers were losing one in five.

Half of those royal children did not reach the age of ten. In a 2024 study, the same research team looked at 124 Habsburg marriages and asked what inbreeding did to the mothers. The more inbred the mother was, the younger she tended to die, and the higher her risk of dying in the four weeks after giving birth. That study cannot tell you what killed either of these two women.

It describes a pattern across the whole family. But set those two women beside that pattern. A mother who died at twenty-one, pregnant again, and a daughter who died at twenty-three, two months after a birth, having already lost two sons. Her son’s story was short too.

In 1698, her uncle Charles, still childless, signed a will naming the boy his heir. On February 6, 1699, the boy died at six years old. Most sources point to smallpox; poisoning rumors were never proven. His death sent diplomats back to the negotiating table.

When Charles died in 1700, Europe fought the War of the Spanish Succession over his throne for thirteen years. In the Imperial Crypt in Vienna, the coffin of Margaret Theresa’s daughter bears a Latin inscription that mentions four things about her: her birth, her marriage, her childbearing, and her death. That was the whole life the family had planned for her. And it was the same life they had planned for the little girl in *Las Meninas*.

The most inbred member of the Habsburg family ever measured was not the “Enchanted King” everyone remembers, the deformed Charles II. It was his niece, the quiet girl from Vienna. Two consecutive uncle-niece marriages did this to a mother and a daughter.

But they were only two stories in a dynasty that kept marrying within itself for generations, and the damage did not look the same on everyone.