Plath is using metaphors to describe how a woman looks during pregnancy. A few examples: a slow-moving house, a large melon with tendrils (arms) sticking out of her sides, a cow in calf, a fat purse with new money inside. As a homage to the expected nine months of a woman's pregnancy, Plath's poem has nine lines and each line has nine syllables.

I'm not sure if I understood my explanation, but the phraseology has stuck with me this past 41 years.