The 39-year-old singer broke the news with a tweet, which read 'I'VE BEEN BURSTING TO TELL YOU SOMETHING ...... :0)', along with a link to a YouTube video.
The two-minute clip then reveals that the pair are to become dads - and not just to one baby, but two!
The video, which is set to the tune of the song 'A Dream is a Wish Your Heart Makes', opens with animated versions of both Ian and Craig. 'Together, we've been on lots of adventures,' they say. 'But our biggest adventure is about to begin!'
'For a long time we have been wishing and our wish is about to come true. We are going to be dads!' An animated baby then appears alongside the pair - but, the video teases, 'there's more... '
The film then ends on pictures of the babies' scans, and whilst the due date is not revealed, viewers are told that the Ryder-Watkins twins are 'arriving soon!'
Watch the heartwarming announcement in full
The happy news is especially poignant as they have been trying to become parents for years, and in 2012 sadly lost a baby that a surrogate was carrying for them through miscarriage.
At the time, Ian told The People that it was the 'worst moment of his life.'
'We were thinking of names, and we even planned our first Christmas as a family. We were both ready to be Dads,' he explained.
'I just feel really empty and helpless. The only person who can help me is Craig.'
'The way I try to look at it is that there's another little star in the sky. It just wasn't meant to be for us at this time.'
Since announcing the recent pregnancy, both Ian and Craig have been inundated with messages of congratulations and support.
Ian tweeted 'Feeling the Love on here tonight!! Thanks for all your messages! #BabySteps'.