Commission for Kassaz. The pose was inspired by an old piece from secretgoombaman12345 depicting Princess Celestia fat as hell: >>810851
This is an underused pose in pregnant pony pictures.
She looks like she’s getting ready for a night on the town. Maternity pants are pants for two or more ponies to wear at once, but colts never wear them again.
This is the base image. More variants are at the source.