I have copied my answer from the other post which I will now delete. (as below)
Every female livebearer (including Mollies)... is pregnant if she's been in the "company" of a male of the same species within the preceeding 3 months..... A general statement, but it is 99.9% true.....
They deliver their fry at (around) 26/7 day intervals -(depending on the water temp) and all fry may be eaten by the mother (or other tank inhabitants) if ample hiding space is not provided for the fry...... Broods may vary between 10 to about 50, with the average (for Mollies) being around 35 but can be up to 100.....Females can deliver a batch of fry for up to 4/5 times after being impregnated by a male.... (she merely hatches the eggs inside of her and "releases" the fry when they've developed.... so they are not "livebearers" in the true sense, but incubators of eggs. The "black spots" you're referring to is called the gravid spot and is an indication of the state of development of the fry inside of her. She will give birth around 2 days after her stomach forms an approximate 90 degree angle with her gillplates. Good luck with yours.
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