Do only pregnant live bearers get a black spot in their stomache?

Hi I just want to know if my swordtail is pregnant? Do only pregnant ones have the black spot in their stomache? I have two that I think are pregnant but I have had them a month now and they had the spot when I braught them but they are fatter then when I got them and I'm sure I can see babies in one of them. I just dont know when they will give birth and what will she look like when she has squared up?