The Mirror's editor, Piers Morgan, refused to divulge the source yesterday, and said no cash was changing hands, though he would be buying "a large Christmas dinner" for the publicist Max Clifford, who had been involved in the later stages of the story.
Mr Morgan said yesterday: "I got the tip myself about six days ago from someone I thought would know. But we couldn't stand it up. Then Max Clifford rang me up and I learnt some more details. And we learnt that Tony and Cherie had told the children.
"But I had to be absolutely sure on this one. This is the Prime Minister's wife and we are The Mirror. It would have been morally irresponsible to publish the story if she had been less than three months pregnant, so I had to be satisfied that was not the case. When I got confirmation of that I decided to publish."
Mr Morgan rang Alastair Campbell mid-afternoon on Thursday only to discover he was at the dentist. "I hoped after that he might splutter and keel over at my phone call," said Mr Morgan.
Instead, Mr Campbell talked to Mr and Mrs Blair, after failing to convince Mr Morgan that he should hold off publishing for a few days. The Prime Minister then spoke to Mr Morgan.
"He accepted it was a big story and we were going to go ahead," the Mirror editor said. "But they would have preferred us to delay. Even Cherie's father didn't know." Mr Campbell, however, did know, and had known for about a month.
The Mirror described the story on its its front page as "Scoop of the Year." The Sun, all too predictably, labelled the same story on its cover a "World Exclusive".
In fact, it was clearly The Mirror's story, though it will be up to the British Press Awards panel next March to decide whether or not it has earned the title it has awarded itself.
Alastair Campbell decided to release the news to all the press shortly before 9pm, thus taking a little gloss off The Mirror's scoop as television news bulletins and the next morning's papers had time to catch up.
But Campbell did a deal with The Mirror, agreeing that he would acknowledge the paper in his statement, which he duly did. The Mirror can congratulate itself on a scoop that will take its place in newspaper history.
However, The Mirror did make one mistake. It wrongly announced that the Blairs' fourth child would be the "first baby in Downing Street" and wrote on the front page: "It is believed to be the first time a PM has become a father while in office." In fact, as was widely reported elsewhere, Lord John Russell became a father at the age of 56 while Prime Minister in1848.
The Mirror was, though, able to give Mrs Blair advice from its own staffer Jill Palmer, who became a mother for the first time at the age of 41. Curiously, she revealed that she was unaware she was expecting a baby for the first three months of her pregnancy. Ms Palmer is the paper's medical correspondent.
Piers Morgan, meanwhile, is revising his opinion that the story is the scoop of the year. "I'm now calling it the scoop of the millennium," he said.