Mr Miliband, 42, and his American-born wife Louise, 45, had previously insisted the matter was private, but in an interview with Readers Digest magazine, he spoke frankly of his love for his family and alluded to the strain of the lengthy adoption process.
In the interview, entitled 'Family Man', Mr Miliband says he would "throw himself under a moving train" for his sons, three-year-old Isaac James and eight-month-old Jacob.
The couple adopted Isaac from the US in December 2004 and were present at his birth. Last October, they repeated the process with Jacob, Mr Miliband withdrawing from a state visit by King Abdullah of Saudi Arabia to fly to the US after his birth.
Mrs Miliband, a concert violinist, has dual US and British nationality, which allows the couple to adopt there.
In both cases, Mr Miliband has refused to give details of the agency which he used for the adoptions and asked for his privacy to be respected.
The Foreign Secretary, who has been tipped as a future Labour leader, hinted that his preparation for the second adoption may have played a part in his decision not to challenge Gordon Brown for the leadership last year.
He said: "The most important thing last year was Jacob. And the last thing I was ever going to compromise on was Jacob.
The couple had considered looking into adopting a child from India or China, he said, but "there were just more points of common reference with America".
That adoption caused a political storm with Tory opponents demanding to know whether Mr Miliband used his position to get the process fast-tracked by the Department for Education, where he was then a minister. It later emerged that his application had taken longer than average to process.
As a 42-year-old he may have faced difficulties adopting in Britain. Adoption agencies say babies in particular are likely to be placed with younger couples.
Mr Miliband said: "We wanted to adopt children as they were born. I think that people who adopt older children are real heroes. Because you are taking on all the difficulties that a child may have had in their early years.
Speaking of his concerns during the adoption process, he said: "Like all parents, you're worried about the health of the child. But then you worry about whether the birth parents will want to go through with it."
The politician said he dislikes the term "giving up" a child for adoption, adding: "Actually we talk about placing your child for adoption rather than giving it up. It is a different thing.
"One's first reaction is, 'Oh my God, how can one do that?' But you realise it is an incredibly loving thing to do.
"We can honestly say to our children, 'Your parents loved you so much – they didn't reject you'."
Mr Miliband said he and his wife can send an annual letter via the adoption agency to their children's birth parents, to tell them of their child's progress, he said.
He added: "Of course, one wonders, is blood thicker than water? But I would throw myself under a moving train for these kids.
"Blood isn't thicker – a child is a child and we are devoted parents. We feel like a complete family now."
The interview is published in the July issue of the magazine.
