"Metaphors" by Sylvia Plath is a 9 x 9 poem (9 syllables; 9 lines) spoken by a pregnant speaker (hence the 9 months).  The speaker uses a barrage of metaphors all of which compare a host-mother to her more valuable cargo-child.

Here are how the metaphors match up:

Host/Mother vs. Cargo/Child

"riddle" vs. 9 syllables"

"elephant" vs. "ivory"

"house" vs. "fine timber"

"melon" vs. "red fruit"

"loaf" vs. "yeast"

"purse" vs. "money"

"bag" vs. "green apple"

"means" / "stage" vs. ("end")

"cow" vs. "calf"

"train" vs. (passenger)

The mother speaker in the poem feels like a "means" to an end, a "stage."  She must feel that people are more concerned about the condition of her baby than they are about her.  "How's the baby?"  "Do you know if it's a boy or a girl?"  "Do you have names picked out?"  She obviously feels unwanted and unappreciated in her pregnant condition.