In this excellent poem by Sylvia Plath, her own experience of pregnancy and her feelings about it through the use of a series of metaphors that are commonly used as the basis to talk about pregnancy in our society. What is important to realise is that this rather unique presentation of who the speaker of this poem is thanks to her state of pregnancy relates explicitly to her identity and the way that the baby, growing inside of her, seems to have displaced her and who she is. Perhaps this is made most clear in the final three lines of this poem:
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.
Note how the metaphors used seem to displace the speaker in terms of her identity. Her status as being pregnant means she is only now a "means" or a "stage" upon which her baby stands. Her ambivalent feelings about pregnancy and motherhood are reflected in the final lines, where her pregnancy is compared to "boarding a train" which she cannot get off. The inevitability of the process is thus refered to, with the way that the speaker mentions that she cannot get off presenting her as feeling trapped or restricted by what is happening to her.