Shamima Begum, who ran away from London with two school friends in 2015, spoke to The Times from a refugee camp where she had fled the collapse of the IS group's "caliphate" in eastern Syria.
Now 19, she expressed no regrets about joining the jihadists but said that two of her children were dead and she was now heavily pregnant.
"I just could not endure any more," she told the newspaper.
She added: "I was also frightened that the child I am about to give birth to would die like my other children if I stayed on.
"So I fled the caliphate. Now all I want to do is come home to Britain."
Begum made headlines around the world when, aged just 15, she left to join the jihadists with friends from Bethnal Green in east London. Another girl from the same school had run away the year before.
One girl, Kadiza Sultana, has been reported killed.
Begum said the other two, Sharmeena Begum -- no relation -- and Amira Abase stayed on in Baghouz, where IS fighters are making their last stand to hold on to the proto-state they declared in 2014.
"They were strong. I respect their decision," Begum said of her friends.
She added: "I'm not the same silly little 15-year-old schoolgirl who ran away from Bethnal Green four years ago. And I don't regret coming here."
- 'Challenge for all of us' -
The British authorities estimate around 900 Britons travelled to Syria and Iraq to join the conflict, of whom around 300-400 have since returned -- and 40 have been prosecuted.
As of last month, around 200 were believed to still be alive and in the region.
Speaking to Sky News, Security Minister Ben Wallace said it was "worrying" that Begum had not expressed regret about going to Syria.
He also noted the difficulties faced by many governments of how to deal with those returning from abroad.
"It is a challenge for all of us," he told BBC radio.
"Some of them were groomed... when they were young but are now adults and some of them are hardened fighters.
"We have successfully prosecuted a number of them in the last few years when they have come back and the others should expect the same if they return."
The United States had said it is ready to help countries repatriate IS jihadists detained in Syria but that ultimately it is up to their home governments to come up with solutions.
Under new anti-terrorism legislation adopted in Britain this week, British nationals spending time in Syria face arrest and up to 10 years in prison on their return.
The law toughens previous legislation that required authorities to prove returning nationals had engaged in terrorist activities while abroad.
A lawyer who represented the families of Begum and her two friends four years ago, Tasnime Akunjee, told The Times he was "thankful she's alive".
He noted that when they ran away, "there was an understanding that as long as they had committed no further offence they will not be prosecuted and become to seen as victims".
- 'My first severed head' -
Begum married a Dutch fighter soon after arriving in Syria. "Mostly it was normal life in Raqqa, every now and then bombing and stuff," she told The Times.
"But when I saw my first severed head in a bin it didn't faze me at all. It was from a captured fighter seized on the battlefield, an enemy of Islam. "I thought only of what he would have done to a Muslim woman if he had the chance."
Begum said she was nine months pregnant. She said her two previous children had died in the past three months -- a daughter, Sarayah, became ill, and a son, Jerah, whose death was linked to malnutrition.
"In the end, I just could not endure any more," she said of her desire to get out.
Begum fled with her husband, but he surrendered to a group of Syrian fighters allied to the US-backed Syrian Democratic Forces.
"The caliphate is over," she said, adding that "there was so much oppression and corruption that I don't think they deserved victory".
She acknowledged her notoriety but said: "I'll do anything required just to be able to come home and live quietly with my child."