By the time Maria Antonia drew her first breath in the Hofburg Palace in January 1669, the family already had a tiny coffin to its name. Her parents’ first child, a small archduke, had died almost a year before her birth—less than four months old, dead a week before what would have been his first birthday. The new baby was a girl, and she would do something her uncle in Madrid never managed: she would grow up, marry, and bear children of her own. By Christmas Eve 1692 she was dead at twenty-three, and two of her three sons were already gone.

Three centuries later, geneticists worked through her family tree and found a number higher than the one produced by incest between a brother and sister. Her name was Maria Antonia, and her pedigree was a labyrinth that closed in on itself. Her father was Emperor Leopold I, ruler of the Holy Roman Empire. Her mother was Margaret Theresa of Spain.

Leopold was Margaret’s uncle—the younger brother of Margaret’s own mother—so the man who held the newborn in Vienna was at once her father and her great-uncle. Through Margaret’s father, Leopold and Margaret were also first cousins. And it went back further still. Margaret’s own parents, King Philip IV of Spain and Mariana of Austria, were themselves an uncle and his niece.

A single couple—Emperor Ferdinand III and his Spanish wife—appeared in the child’s family tree twice: once as her grandparents on her father’s side, and once as the grandparents of both her parents on her mother’s side. The family had done this on purpose. Marrying within the family kept Spain, Austria, and every claim attached to them in Habsburg hands. With each generation, the circle of people allowed to marry one another tightened a little more.

And in this story, the first to pay the price was the baby’s mother. When Margaret was five years old, in 1656, Diego Velázquez painted her standing in a room of the royal palace in Madrid. That painting is Las Meninas. Look behind the small blonde girl, at the mirror on the back wall, and you can see her parents: the king and his niece.

Four years later, at age eight, Margaret was betrothed to Leopold. Over the following years, her portraits were shipped from Madrid to Vienna so the emperor could watch his bride grow up in paint before he ever met her in person. She mattered far beyond Vienna. When her father died in 1665, his will named her next in line for the Spanish throne if her frail younger brother Charles did not survive.

In April 1666, at fourteen, she married Leopold by proxy in Madrid, with someone else standing in for the groom. Afterward she sailed from the Spanish coast to Italy, crossed the Alps, and met Leopold for the first time outside Vienna in late November. In December 1666, a week after she entered the city, they married in person. She was fifteen and he was twenty-six.

The court made it look like a fairy tale. For her seventeenth birthday, Leopold staged a grand opera in her honor, Il pomo d’oro—The Golden Apple. In private, she called him “Onkel,” the German word for uncle, and he called her “Gretl. ” By most accounts, the marriage was happy.

That was the story on the stage. The nursery in the Hofburg told a different one. First came a son, born in September 1667, who lived about three and a half months before ending up in that coffin. Then Maria Antonia arrived in 1669 and was the one who survived.

In February 1670 a boy named Johann Leopold was born and died the same day. In February 1672 a girl lived only two weeks. Add two miscarriages, and Margaret had gone through six pregnancies in roughly six years: four dead infants and one survivor. That was close to normal for her family.

In 2009, geneticists at the University of Santiago de Compostela studied the children of the Spanish Habsburg kings and found that between 1527 and 1661, half of them died before age ten. Margaret had been one of very few children of her father who reached adulthood. Now she watched the same thing replay in her own nursery, and she had to carry every one of those pregnancies. In March 1673, four months into another pregnancy, Margaret died in the Hofburg Palace.

She was twenty-one. They buried her in the Imperial Crypt in Vienna, and her four-year-old daughter became all that remained of the marriage. Leopold did what the system required, and he did it quickly. Seven months later he remarried—another Habsburg, Claudia Felicitas of the Tyrol branch.

But in a letter he wrote that summer, on what would have been Margaret’s twenty-second birthday, he wrote that his new bride was not “like my only Margareta. ” Leopold’s later nursery makes the comparison hard to ignore. Over the following decades he fathered twelve more children with his later wives, and six of them lived to adulthood. Compare that with Margaret’s single survivor out of four.

Now consider what that meant for the four-year-old girl in the palace. With her mother dead and her uncle in Madrid still childless, the child suddenly became something far more dangerous than an archduchess. In Spain, the royal court treated her as the most direct heir to her uncle’s crown—the entire Spanish Empire, including its overseas colonies. But her father did not want anyone viewing her that way.

Leopold considered himself the rightful heir to Spain, and later he wanted the throne for a son from his third marriage. So he kept her claim to the throne secret. The historian who studied her portraits, Rocío Martínez López, found that the paintings made of her in Vienna show an ordinary archduchess with no hint that she stood next in line for the Spanish Empire. In Madrid, her grandmother Mariana wanted the exact opposite.

She wanted Margaret’s descendants to inherit Spain. So the child in the Hofburg became a piece pulled in two directions by two branches of her own family. She grew up inside that struggle. She was a serious girl who loved Viennese court music and became one of its patrons while the adults around her argued over her worth.

One husband they considered for her was her uncle Charles himself, the King of Spain. Had it happened, it would have been the third uncle-niece marriage in three generations of the same dynasty. In the end, that marriage never took place. Leopold had a better plan for using her.

On 15 July 1685, at sixteen, she married Maximilian II Emanuel, Elector of Bavaria, a twenty-three-year-old military commander, in Vienna. The deal came at a heavy price. Her father made her renounce her rights to the Spanish throne. In return, her new husband was promised a major post: governor of the Spanish Netherlands.

That promise would come back to haunt her life at the worst possible moment. Within a few years, behind closed doors, the marriage was in real trouble. Her husband was outgoing and restless, a soldier first and foremost. She was quiet and serious.

Surviving accounts record that he was constantly unfaithful to her and that she took it as a personal insult. At the same time, the court expected an heir from her, since an heir was the entire point of the marriage. She became pregnant and gave birth to a son. He did not live long.

Then the same thing happened with a second son. Most accounts record that both boys died at birth or very shortly after. The precise dates of those two births rest on weak sources. What everyone agrees on is that two sons came before the one who lived, and neither of them survived.

Here is the detail that is easy to miss. It is her mother’s story repeating itself. A daughter and niece losing one baby after another in the cradle while still in her early twenties. Then came time to cash in her father’s promise.

Her husband got his post as governor and left for Brussels to administer the Spanish Netherlands. She was pregnant for the third time. When her time came, she was not in Munich or Brussels. She had gone back home to Vienna.

What happened in Vienna that autumn is why this story needed to be told. On 28 October 1692, in the Hofburg Palace—the same building where she had been born—she gave birth to a boy. They named him Joseph Ferdinand. He was healthy and he survived.

For a few weeks, it seemed she had finally accomplished the one thing everyone around her wanted. Then, in December, she fell ill. The account most writers repeat today is puerperal fever, an infection no physician in Vienna could treat at the time. On Christmas Eve, she died in the palace where she had been born.

She was twenty-three years old, and her son was eight weeks old. On Christmas Day they placed her in the Imperial Crypt beside her mother. Their coffins are still there today, in the crypt below the Capuchin Church in Vienna. Two women, mother and daughter, both dead in their early twenties, both after years of pregnancy, both leaving a single living child behind.

For three hundred years, no one could put a number to what had happened to them. In 2013, two researchers from the same university in Santiago—Francisco Ceballos and Gonzalo Álvarez—published a study in the journal Heredity. They reconstructed the Habsburg family tree from 1450 to 1750, thousands of people across more than twenty generations, and calculated what geneticists call the inbreeding coefficient for the entire dynasty. Here is the easiest way to think about it.

Pick any gene in your body. You have two copies, one from your mother and one from your father. The coefficient is the probability that those two copies are actually the same copy, passed down through two branches of one family and meeting again in you. For a child of a brother and sister, that probability is 0.

25—one in four. In an earlier 2009 study, the inbreeding coefficient of the Spanish kings had risen from 0. 025 for the first of them to 0. 254 for Charles, the king everyone points to as the worst case.

That same study found that inbreeding, even no more than between cousins, reduced the chance of a royal child reaching age ten by about 18 percent. Margaret had the same parents as Charles. So theoretically, she carried his number too. Maria Antonia reached 0.

305. That is not a typo. In the 2013 paper, it was the highest in the entire dynasty—higher than her uncle, higher than her mother, higher than that one-in-four number. No brother-sister marriage anywhere near her in the family tree produced it.

It happened because of two consecutive uncle-niece marriages, stacked on top of earlier cousin marriages that kept reinserting the same ancestors. Still, the number 0. 305 has limits. It is only a calculated figure in one paper, based on the family tree, not on her DNA.

As far as anyone has found, no higher coefficient has been measured for any royal with a well-documented family tree. Older dynasties may have gone further, but their records have gaps too large to make the calculations equally reliable. That leaves the question. If her number was higher than Charles’s, why did she grow up, marry, and bear a child who lived, while he could do none of those things?

A high coefficient makes it likely that two damaged copies of a particular gene will meet in a child of the family, and chance decides which child. Charles got a body that failed him in public. Maria Antonia and her mother got bodies that looked sound in the records but failed them in the one place royal women were judged: the nursery and the birthing room. In 2025, the same group—now joined by Roman Vilas—looked specifically at Habsburg mothers.

The higher a Habsburg woman’s inbreeding coefficient, the more likely she was to die in the weeks immediately after childbirth. Maria Antonia died about two months after her son’s birth, which is later than the range that study measured, so it cannot prove what killed her. But it places her death within a pattern the researchers could actually count across the family. And that brings the story back to the one child who survived.

That boy was about to become the most important child in Europe. Charles had never fathered children, and the great powers worried about what would happen when he died. If Spain went to a French Bourbon or an Austrian Habsburg, one of the two families would become far too powerful. The boy was the way out.

He was Margaret’s grandson, but he was Bavarian—neither French nor Austrian. In 1698, England and France signed a treaty naming him heir to the Spanish throne and granting parts of the empire to France and Austria. Charles refused to see his empire divided. That same month, November, he remembered the boy in his will and left everything to him.

The grandson of the little girl from Las Meninas, the boy whose mother died weeks after his birth, was going to become King of Spain. The timing of what happened next was cruel. On 6 February 1699, the boy died. He was six years old.

The usual cause given is smallpox. Rumors that his grandfather Leopold had him poisoned were never proven. With his death, the last living line of Margaret ended. Four infants, one daughter, three grandsons—and by 1699, no one was left at all.

Charles died the following year, in November 1700, leaving everything to Philip, Duke of Anjou, grandson of King Louis XIV of France. Austria refused to accept it, and Europe went to war over Spain for more than a decade. By some estimates, hundreds of thousands of people died. For nearly two hundred years, the Habsburgs had intermarried to keep Spain in the family.

It ended with the crown of Spain placed on a French head. And the girl at the center of Las Meninas was left without anyone to carry her line. The most inbred princess ever recorded had paid the price for her family’s plan.