Lett, a physiotherapist and Pilates instructor, said yesterday was one of the best days of his life.
He told the Herald his wife, also a physiotherapist, Pilates instructor and keen runner, had suffered "some pretty uncomfortable" contractions all yesterday as she and a group of friends drove from point to point around the course of the marathon to support him.
Though his wife was not due to give birth until next week, Lett said, his mind was on her during the race, and the thought of the baby coming early had crossed his mind.
"It wasn't expected, but it had been joked about a few times. At least it was after the race that it happened."
Staying in the leading group for the whole marathon, Lett finished in first place in a time of 2h 27m 38s, two and a half minutes shy of his personal best of 2h 24m 58s - set last year when he came second.
Lett's coach, Sam McLean, said he was waiting for the prizegiving to begin when the runner told him he had to leave.
"Steve came up to me and said, 'We're off.' I said, 'Where are you going?' and he said, 'To the hospital - Felicity's having a baby'."
Mr McLean accepted the prize on Lett's behalf and said a cheer erupted when he explained to the crowd what had happened.
The ASB Auckland Marathon began in Devonport at 6am.
Stephen Lett crosses finish line in first place. Photo / Greg Bowker
Participants made their way through Devonport and Takapuna before crossing the harbour bridge and heading to St Heliers, then turning to head for the finishing line at Victoria Park.
Wellingtonian Nick Horspool finished a minute behind Lett, in a time of 2h 28m 46s. In third place was Aucklander Josh Maisey, with a time of 2h 29m 8s.
Beijing Olympian Liza Hunter-Galvan of the United States won the women's marathon in 2h 47m 42s, while last year's winner, Australian Kirsten Molloy, came in second more than 10 minutes behind. Third place went to New South Wales' Melissa Criniti with a time of 3h 4m 26s.
Hamish Carson and Camille Buscomb won the half-marathons.
About 17,000 people participated in the day's events, which also included a quarter marathon, a 5km challenge and a kids' "marathon".
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