Winslet, 25, was pregnant during the filming, and inevitably gained weight.
"I just got so fat, so we used to strap everything in to make it not show," she told reporters. "I was about six months pregnant by the end."
Enigma premiered at the Edinburgh Film Festival on Saturday.
The film, which is based on Robert Harris' best selling novel, centers on cryptoanalysts at Bletchley Park, Britain's top code-breaking center, after Nazi U-boats have unexpectedly changed the code by which they communicate with each other and German High Command.
In the World War II spy thriller, Winslet plays Hetser, one of Britain's code-breakers. The weight gain, she noted, suited her character.
"I did look quite podgy which kind of worked with a boffin," she said.
Mick Jagger, who produced the film along with Lorne Michaels, was noticeably absent from the premiere. Jagger, who has always had a passion for spy stories, has apparently made a smooth transition from rock 'n roll arenas to the silver screen.