Andie identified the person who wrote the original essay outlining the argument, MIT's Judith Jarvis Thompson.
In it, the essayist deals with what you say -- that it does indeed grant the "personhood" of the dependent being you are suddenly woken up & attached to. In fact, Judith at the outset says, (paraphrasing) -- okay, fine, let's *concede* the personhood of the dependent being in the thought-experiment; we'll take a different a tack, then, and still show why pro-lifers arguments are still ethically wrong.
The attitude of your argument seems to be the very Catholic one of, "You had sex, so now, ha, pay the consequences."
-B.
Michael Smith wrote:
"It grants the opposition's core premise -- the personhood of the fetus. My personal shorthand for this is 'yes-buttery': the liberal's characteristic response to any reactionary argument always opens with the phrase 'Yes, but...'"