The singer is set to welcome her first child with Belafonte this summer, joining her former bandmates Victoria Beckham and Emma Bunton.
She revealed the news in British Hello! magazine and said that the couple are 'really thrilled'.
Brown said: 'It was planned, obviously. When you're 35 years old, you have to do your check-ups beforehand. So I'd done all the precautions around Christmas time, then we started trying.'
She added: 'We both really wanted things to happen. The power of wanting a baby, I think, can sometimes just work for your advantage.
'I didn't think that I would get pregnant that quickly. Usually it takes at least a month or two.'
The new baby will join the 35-year-old's daughters Phoenix and Angel Iris, as well as Belafonte's daughter Giselle.