I'm a riddle in nine syllables,
An elephant, a ponderous house,
A melon strolling on two tendrils.
O red fruit, ivory, fine timbers!
This loaf's big with its yeasty rising.
Money's new-minted in this fat purse.
I'm a means, a stage, a cow in calf.
I've eaten a bag of green apples,
Boarded the train there's no getting off.
The speaker feels that she is a “means” for someone else (ie. a means to an end— the end being the baby’s life).  She is also an everyday means of sustenance for the baby. Her body is not her own; it has been repurposed as a staging ground, expectantly awaiting the performance, or birth. The word of course also suggests a stage of life. 
  The idiom “in calf” means to be pregnant.  Probably a stretch to say it’s also a punny mock of her swollen ankles.
Metaphors