Large Pregnant Female Guppy

The first thing to check, just in case (I always like to cover all possibilities no matter how remote) - does this female's scales stick out when viewed from above? It's only a vague possibility but you need to rule out dropsy rather than hanging onto fry as a cause of her size.
Is she almost square viewed from the side? Most livebearer females 'square off' a couple of days before dropping, I presume the others did?

I have 3 adult female endler-guppy hybrids, and to be honest I've never noticed any particular thing that triggers them to drop fry. I know other members have platies and mollies, perhaps they can suggest what works for them.

Hi Sue - just an update on my recent post, all well and I think she is dropping fry quite slowly looks like a few a day as she is reducing in size although still quite big for a female guppy. Ive probably got now around 30 or 40 fry all feeding well on microworm and crushed flake. I need to start thinking now about separating the males from the females as I know they are sexually active after 6 weeks not quite sure. I have 2 set ups 1 of which is for males and the other one for females,both with the same water quality.  I have 7 females in total all quite big... the fry at the moment are around 3 weeks old and already I can see colouration starting to develope everyday. I just need to highlight the males and females at this stage - how do I recongnise them lol   paul