All of "the mother" is a direct address to an audience. The mother sometimes speaks to her reading audience, sometimes she speaks to herself, sometimes she speaks to her non-existent children. These changes of address have a huge effect on the poem. When she speaks to her readers, the mother effectively implicates us in her life—particularly, in her decision to have an abortion. When she speaks to herself, we feel like we are overhearing her working through some serious issues. And when she speaks to her non-existent children, it's almost as if she calls them to life—she imagines that they are real, live, human beings. Brooks accomplishes so much with just a few shifts in address.
Lines 1-10: Here, the speaker addresses "you," which is to say, us (her readers). By doing this, she speaks about the effect of abortions generally, not specifically. It's like what she's saying applies to everyone who has had an abortion.
Lines 11-14: In these lines, the speaker uses the word "I," and seems to be taking to herself now. She's contemplative, and she's turning things over in her mind.
Lines 15-32: In the second half of the poem, the speaker addresses her non-existent children directly. She calls them into being with just her language, but this has the strange effect of drawing attention to the fact that they don't actually exist. There's a lot of power in the direct address.