First of all, pregnancy is overwhelmingly a choice (of course there are exceptions)--it's not our hypothetical organ donor's fault that the hypothetical recipient needs an organ, but it is the parents' "fault" that an unborn child (recall we're not debating point-of-life at this point in the thread) needs to be born insofar as the child's life is the result of a risk the parents knowingly entered into (again, there are exceptions).
Secondly, if our hypothetical organ donor backs out, our hypothetical recipient can get an organ from another donor--with few exceptions, there's no way to transfer a pregnancy (at least not outside of some extremely early window).