She hasn't exactly come out and verbally admitted that she's pregnant, but after seeing Zoe Saldana's growing baby bump, it's pretty clear that she's not carrying around a food baby.
For an appearance on Good Morning America to promote her new movie, Guardians of the Galaxy, Zoe showed up in a fitted maroon dress. It basically screams, "Yes, I'm pregnant, ok? But it's nobody's business but mine, so I don't feel the need to talk about it."
Have a look at her bump and see if it looks like she could possibly be anything other than pregnant.