How do you really know if a livebearer is pregnant?

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Sure you see a big belly & gravid spot, but I've seen that in my platies & they never give birth. I really want to know which of my platies are pregnant. I got some today and one has a big stomach & gravid spot but my platies & mollies that I've had before were always like that. How can you really tell? Is there some way...?
 
one sign is that the gravid spot will almost look like a solid black patch.
2nd thing that u shud notice b4 she about 2 give birth is when she keeps swimming away from the others and she may look like disoriented.
 
Livebearers can re-absorb babies if they feel conditions are not right. Do you have enough room in the tank free from predators? Plenty of plant cover for security?
 
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