The pearlized G1 babies are the ones that were made by Majesty's
unholy enchanted mirror. Hence why they're shimmery, dead-eyed, and sometimes slightly "off" from their moms like how Surprise is yellow, Moondreamer has a different name, etc. Turns out, an ancient mirror in a dark room isn't that great at making perfect copies.
"Regular" P&C-type babies in my herd are usually baby versions OF the ponies (à la Muppet Babies) and only rarely are the offspring of the adults, while "original" babies (e.g. Noddins, Sunribbon) were made the old fashioned way.
G2s aren't actually closely related to the G1/G3 pony clade -- their similarities are due to convergent evolution.
Core 7-era G3 was a terrible magical mistake that occurred because Pinkie Pie extorted an unfortunately-worded wish out of one of the last G1 sea ponies, decimating the population. (A mistake which only got compounded into the horrors of G3.5, in the style of the fisherman's fable.)