Livebearers can retain sperm for 6 months or so after being with a male, and impregnate themselves when the time is right
. Mollies and platies can not breed, but platies and swords can. If you really want to try and save every fry you can put her in a breeding trap when she starts looking "square". It's probably easiest to just let "nature take it's course", leave the female in the tank, some fry might survive some or all might not.