Renee Duering still remembers how painful it felt when an Auschwitz prisoner tattooed the camp number on her arm.

"Be glad you're getting a number; otherwise, you'd end up straight in the oven," the man told her.

The Nazis gave her a choice: "Either you go to the Birkenau extermination camp or become a subject for medical research. That won't kill you.”

Duering, who was born in 1921 in Cologne, chose the latter, becoming a test subject in the hands of Nazi gynecologist Carl Clauberg.

She was one of hundreds of Jewish women who were subjected to sterilization experiments, and in 1992 told her story to the United States Holocaust Memorial Museum. She died in 2018.

Women and children await their fate at the Auschwitz camp in Birkenau. Some would be spared as human experimentsImage: Yad Vashem Photo Archives/AP Photo/picture alliance Carl Clauberg's insidious hormone research

Clauberg studied medicine at Kiel University and received his doctorate in 1925. He specialized in gynecology and worked with chemists from the Schering-Kahlbaum pharmaceutical company to develop hormonal compounds. His method of helping infertile women get pregnant established him as an authority on hormone research.

On May 1, 1933, Carl Clauberg joined the National Socialist German Workers' Party (NSDAP) and the SA ("Sturmabteilung," a paramilitary combat organization of the NSDAP). Like many doctors in Germany at that time, he hoped that Nazi leadership would help him advance his research. Under the regime, every German woman was expected to have as many kids as possible — preferably blond and blue-eyed.

But Clauberg also conducted research on how to sterilize women. This supported the inhumane, racist stance of the Nazis, whose objective was the extermination of Jews, Sinti and Roma people, and other marginalized groups such as gay and Black people and the disabled.

Carl Clauberg: Hundreds of Jewish women were victims of his sterilization program Image: www.auschwitz.org The hell of Block 10

In 1942, Clauberg sent a request to Heinrich Himmler, the chief architect of the Holocaust and the second most powerful Nazi after Adolf Hitler. Clauberg said he needed facilities to implement his "new method of the non-surgical sterilization of inferior women."

By the spring of 1943, the doctor was not given his own institute but was allocated a block in Auschwitz. There he set up his very own experimental laboratory in Block 10.

The first Jewish women from the neighboring Auschwitz-Birkenau extermination camp were transferred there.

Clauberg himself admitted that the female prisoners were faceless; he was only interested in their lower abdomens. Years later, Renee Duering recalled the torture she had endured at his hands.

"We were taken one by one into a room and laid on a black glass table, which was an X-ray table. While the liquid was being injected into our bodies, the X-ray machine was running so that the doctor could see what was happening … the injection burned so horribly."

Neither Renee nor the other women knew at the time what was being done to them. Previously, Clauberg had only ever conducted these experiments on animals. His instruments were not sterile and Clauberg would use them multiple times. There was also no anesthesia — only the injection.

Depending on the state of the fallopian tubes, the doctor might inject a toxic substance into the abdomen of his subjects, which glued the walls of the fallopian tubes together and burned them closed in the process. If this didn't work, the procedure was repeated. "I had to lie there for three days in terrible pain," Duering recalled.

Common side effects from Clauberg's experiments included pus-filled peritonitis — swelling of the belly — blood poisoning, labor-like pains, and terrible burning sensations. The women tried to hold back their screams, because they knew that if they were heard they would be sent to the Birkenau gas chambers.

A syringe from a hospital in AuschwitzImage: www.auschwitz.com Human guinea pigs 

How is it possible for a doctor to ignore ethical concerns and treat people like animals?

"Medical and human considerations come to play a secondary role once someone has concluded that they were no longer human beings, but subhumans," historian Andrea Löw from the Center for Holocaust Studies in Munich told the Neue Osnabrücker Zeitung.

Löw said that in the case of Clauberg, "boundless ambition” also contributed. "He saw his chance to take advantage of the system to advance his career and achieve fame and glory. He subordinated everything else to that."

Himmler asked Clauberg how long it would take to sterilize 1,000 women. The doctor replied that a suitably trained physician working alone with 10 assistants should be able to sterilize a few hundred, if not 1,000 Jews in a single day.

The failure to get justice

But he never had the opportunity to conduct industrial-scale sterilizations. On January 27, 1945, the Red Army liberated Auschwitz. Clauberg had already fled to the Ravensbrück women's concentration camp, where he continued his experiments.

Once the Soviets closed in on Ravensbrück in April, he fled again. Two months later, he was found, arrested, and sentenced to 25 years in a penal camp in Moscow.

Holocaust survivor Margot Friedländer dies at 103

To view this video please enable JavaScript, and consider upgrading to a web browser that supports HTML5 video

But in 1955, he was released early. According to the files of the Kiel Public Prosecutor's Office, he felt that he was given a "royal welcome in his home town."

Clauberg returned to work at the Kiel University Hospital. Because the medical profession was still far from being denazified, a colleague who had worked in Auschwitz was more than welcome.

But in November 1955, the Central Council of Jews filed a complaint against Clauberg, with over 100 witnesses willing to testify against him. He claimed that he was the victim of slander and felt that he had been wrongfully accused.

According to the investigation files, Clauberg claimed that he had wanted to save the women in the block from certain death in the gas chambers.

But Carl Clauberg, who sterilized somewhere between 500 and 700 women, hadn't yet been brought to trial when he died on August 9, 1957.

Pill arrives in 1960 thanks to human experiments

Many of the doctor's victims continued living with the trauma and infertility. Renée Duering, however, miraculously gave birth to a daughter, despite Clauberg's horrific interventions.

On August 18, 1960, the first hormonal contraceptive drug known as Enovid was launched in the United States.

Enovid birth control pills helped spark the sexual revolution in the 1960s, yet had sinister originsImage: Everett Collection/picture alliance

Research conducted by Clauberg contributed significantly to its development. The Schering company, which had financed Clauberg's experiments, was absorbed into the Bayer pharmaceutical group — which still markets the contraceptive pill today.

"This revolutionary method of family planning became a key factor in emancipation and a turning point for society," the company proclaimed on its website.

But the women in Block 10 did not have the freedom to decide on motherhood.

This article was originally written in German.