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6 November 2018, 10:17 | Updated: 23 March 2019, 23:26
The TV star has already picked out a name for his third child.
Announcing the news in OK! magazine, Gregg and wife Anne-Marie revealed that she became pregnant after two years of trying for a baby.
The MasterChef judge said they had been due to start IVF, but they were surprised by the "miracle" of having naturally conceived.
He told OK! magazine: "I've been scared and stressed since she fell pregnant but since we had the 12-week scan recently it feels joyous.
"Seeing a scan of our baby son and the strength of his heartbeat made me cry."
The baby boy is due to arrive in May next year, and Gregg said they've already picked the name Sid Massimo Wallace.
Gregg added that this will be his last child, as he'll be "too old" to raise any more children.
"Usually when you have a child, by the time they’re 20 the parents get their life back a bit, but I’ll be in my 70s.
"So, we agreed; one child and she’d have some help here at home – so her mum and dad have moved down from Coventry and they share this house."
Anna added: "I've been happy with the idea of just having one child right from the start. The fact we’re having one baby is a miracle - we're blessed enough."
Gregg, 54, married his 32-year-old wife in 2016. He has previously been married to three other women, and has two teenage children from his second marriage.