The pair already has three sons - Brooklyn, 11, Romeo, eight and Cruz, five - and they have revealed they do not know the gender of their imminent new arrival.
Beckham has now sparked rumors the couple will try for another baby in the future, after telling CNN's Alina Cho, "(If I) don't have a girl this time, then maybe I'll be lucky enough and have a girl the next time."
But she is adamant five children will be her limit - because she can't fathom having as big a family as Angelina Jolie, who has six children with partner Brad Pitt.
Asked if she would have a fifth baby, Beckham replies, "There could be. Who knows? But a sixth? Well, I'm not Angelina. I don't know. I mean, come on."