Williams, 38, said he and wife Ayda, 32, were moved to tears when they saw the scans.
He later blogged: “I’ve been keeping a secret from you all – me and Ayda are going to be mummy and daddy this year.”
The couple were married in August 2010. Williams, who rejoined Take That to tour last year, has long talked of wanting to become a father and became an ambassador for children’s charity Unicef just over a decade ago.
Friends and fans offered their congratulations yesterday, and his friend Jonathan Wilkes said how difficult it had been to keep quiet about the pregnancy.
In a message on Twitter, he said: “That’s the biggest secret I’ve ever kept. So proud of my mate and Ayda.”
Among the singer’s albums is one aptly titled, I’ve Been Expecting You.