"I have always imagined three faces around the dinner table, and I just couldn't move on," Mrs Bridges told NZME.
The baby's due date is December 18 - two months after the general election.
It would be perfect timing, they said, because it coincided with Mr Bridges' Christmas break from Parliament and would allow him to be more hands-on this time round.
Natalie Bridges said the couple were not pinning hopes on any one gender,
"We think we're very lucky to be having another baby, so we're not worried."
The baby-to-be's brothers are already excited,
"We told Emlyn and Harry and they were so excited - they think the baby is a Christmas present because we said he or she will be here by Christmas"
The family plans to stay home for the holidays.
"I'm not sure what tasks I'll be assigned, but it will be awesome to be here for a longer period of time at the start, something I didn't have so much with Emlyn and Harry," Mr Bridges said.
His wife said: "It will be wonderful to have Simon around for this little one. He's always had to be back in Wellington soon after, but this time he will be here, which will be lovely."
As well as MP for Tauranga, Mr Bridges is Minister for Economic Development, Minister of Transport, Minister for Communications, Associate Minister of Finance, and Leader of the House.
He is in Wellington during the week and Mrs Bridges runs Tauranga PR and marketing agency Blink, which she founded when Emlyn was 6-months-old.
"It's always a juggling act. But I love working and I managed to work and look after my other two children. This child will be the same. I think you just find ways to run life as best you can when you're a busy working mum," she said.
"We are very organised at home and the children are used to routine but are also great at going with the flow - you need both in my opinion to be able to keep things running happily."
Mrs Bridges is from Britain, and her parents are scheduled to fly out to help in February.
