The couple - who announced Field's pregnancy last week - were recently rumoured to be having a baby girl.
Williams and Field were also said to have chosen a shortlist of baby names, including Roberta, Genevieve and Alice.
However, the singer posted on his official website that he and Field did not know the baby's gender and that they have not decided on any possible names.
"To fill you in we don't know if it's a girl or a boy," he wrote. "We've no names yet either. Maybe you can give us a few ideas?"
Robbie Williams's bandmates Gary Barlow and Mark Owen are also expecting children later this year.
Watch the video for 'Bodies' by Robbie Williams below: