"I was shocked and elated," she tells PEOPLE and Entertainment Weekly editorial director Jess Cagle. "And then I was like 'Hold on, wait a minute.' "
In her new memoir Love, Loss and What We Ate, excerpted in this week's PEOPLE, Lakshmi reveals how soon after she got the news, "a question began to nag at my giddiness: not so much 'How?' but the far more uncomfortable 'Who?' "
But the truth, she writes, "was that I didn't know the paternity myself, until that September."
At the time she became pregnant, Lakshmi was dating IMG CEO and billionaire Teddy Forstmann and Adam Dell, the brother of Dell Computer founder Michael Dell. She says she was up front with both men, telling them she was dating other people. Although she had since broken up with Dell that February, she writes, "We had fallen back together briefly in June."
Watch more of The Jess Cagle Interview with Padma Lakshmi on People.com all week
When she told Forstmann the news, he reacted with anger. "I saw his face go white, then beet red," she writes. "It was as if the room could not contain him, or as if his body could not contain his fury at the information he had just received."
After he took a paternity test that revealed he was not the father, Forstmann, "implored me to keep the results between us," she writes, offering to raise the child as his own, but she could not agree.
To read an except from Padma Lakshmi's new book, Love, Loss And What We Ate, pick up this week's issue of PEOPLE magazine, on newsstands Friday.
Then, at a time when Lakshmi expected him to leave her, he accompanied her to her sonogram and said, pointing to the screen: "This is all we need to think about. This is all that matters."
Looking back, Lakshmi says, "The fact that it was public was excruciating because it's a very private matter but that was nothing compared to how I had hurt Teddy."
Lakshmi dedicates her memoir to him and says the fact he stood by her, "was a great example of Teddy's character."
"He wasn't going to leave my side," she says. "Maybe he didn't know if he could stay in a romantic relationship with me or not. I think neither of us knew what was going to happen but he was resolute in making sure I wasn't alone. He held my hand and he held it very publicly."
Of her book's candor, she notes: "There were a lot of difficult things I went through in a very short intense period of time under very public circumstances and I needed to be forthright and honest about it. I needed to write the kind of book that I would read, frankly. I wouldn't want to read a book that was just one long fluff piece."