i assume stressful, of ocurse i dont know for a fact.
the thing is, i been keeping platies and guppies etc for years, sometimes you can think if a fish is about to "pop" and it still takes 2-weeks for them to give birth!!! and sometimes, they might give birth and you didnt even know they were preggers
So its hard to tell....
What you may find when she is about to give birth (if shes in main tank) is that she will hide away, almost like shes trying to get some privacy!
On the other hand, if its obvious she's giving birth you might find 5 fish chasing her, sensing a nice extra meal is about to come their way!!!