The 35-year-old star - who is expecting her second child with her fiance Jade Jones - doesn't see Victoria Beckham, Geri Halliwell, Mel B or Mel C as often as she would like, but says they grow closer when one of them is going through a life-changing experience.
She explained: "Because we're all so busy we don't get to see as much of each other as we'd like but when something like this happens the bond we have shines through. It's like a sisterly relationship and that will always be there."
The blonde singer, who already has three-year-old son Beau with Jade, also revealed the former bandmates use family occasions as an excuse to get together and socialise.
She said: "In fact Scarlet [Mel C's daughter] and Cruz [Victoria and David Beckham's son] have birthdays in a few days so it's great excuse to get together."
Emma is particularly close to Victoria and David - who are currently expecting their fourth child - and she recently enjoyed celebrating her engagement with the couple.
She said: "Because Victoria and I are pregnant we talked about that for a bit then I said, 'I do have some other exciting news' and I held up my ring. They jumped out of their seats and hugged and kissed us."