I have commercial types, mules, suffolks, Southdown x, and they definitely change shape about 2 months from due date but last year udders developed really early and vulvas really pink and swollen almost 8 weeks!
They don't seem so visible this year bar one first time Wiltshire Horn who has been swollen for several weeks and udder starting to swell. She isn't due for at least 2 weeks....I hope!