They are definitely pregnant, the black spot (gravid spot) will always show in a fish but the larger and darker it is (like your fish) the more likely she is pregnant. When she gives birth her and the others will try to eat the fry (babies) so you need to give them plenty of places to hide or escape the larger fish.
Just when you think she can't get any bigger without exploding she shouldn't be more than a day or two away from birthing, she will go off her food and hang out away from the other fish maybe by the heater or filter if possible.
The fry can be fed various things, flake food crushed really fine so its like a dust, some people use boiled egg yolk mixed with water to make a sort of watery paste or you can buy a livebearer food called liquidfry. Because their stomachs are so tiny you need to feed them a little at a time but often, I feed new fry upto 4 times a day as they get bigger I drop it to 2 but still feeding small amounts
hope this helps