Pregnant platy?

Question
I know I keep on emailing but I'm impatient, sorry.
One of my platy females has a rather large belly and what looks like a small cluster of dark spots in her belly, just in front of her anal fin. At first I thought it was poo, but the large black spot has been getting larger over the last few weeks and when I leave her to digest her food, the spot doesn't go.
She is very territorial and I think she was fertilised before I got her.
Her belly is quite large and doesn't shrink when she poos like all the other fish do.
Her belly is very slightly bigger than the others. when will she give birth, depending on the water and food conditions?
I also want to ask, do some platys, when pregnant, ALWAYS have a large black spot on her belly? Because one of my other females has a large belly, it doesn't shrink now and I can't see any spot, is this normal?
Can male platys tell if a female is fertilised? Because he is only chasing one of the females, before he was chasing all.
Please help.

Answer
Hi Rhia,
 I do not know of any way to tell exactly when a platy will give birth.  Basically, they get really large bellies and then they give birth.  You just have to watch them closely.  

-- Ron C.
  rcoleman@cichlidresearch.com
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