Initially, the speaker describes her negative feelings about the fact that her birth was planned. The image of her mother taking the "cardboard out of [her father's] shirt from the laundry" so that she can use it to chart her basal temperature feels rather emotionless and commonplace.
Further, the simile in which she compares removing the cardboard to "sliding the backbone up out of [her father's] body" is odd and even sort of medically off-putting. The speaker would rather have been conceived in "heat" rather than in this cold sort of way: using cardboard and numbers and dates.
However, one day, drinking wine with a friend, the friend points out to the speaker that she must have been wanted. Suddenly, the "wine against [her] lips" makes her think of moving down her mother's birth canal; this simile is close and wet and tense, the repetition of the phrase "bearing down" and the image of being "press[ed] . . . out into / the world." Instead of regretting that she was not conceived in "heat" now, the narrator seems to realize that, for her mother, the little cardboard calendar was created because not a single incredible and beautiful part of the world could make it sufficient without her child. Suddenly, the funny little calendar and her mother's careful "little x[es]" seem like such a labor of love (no pun intended).
In the end, then, the speaker seemingly learns to think of her conception differently: it becomes something beautiful, her mother's dogged devotion and longing for her, rather than something clinical and cold. The speaker learns that she is more important to her mother than anything else. We might interpret this poem's purpose as having to do with the idea that children often do not understand their parents' choices. It might compel us to consider our parents as individuals with their own wants and needs rather than how their choices have impacted us.