Repeal of birthright citizenship for the US-born children of unauthorized immigrants would expand the unauthorized population at least 5 million over the next four decades. Employing standard demographic techniques, this analysis suggests that there would be 4.7 million unauthorized immigrants as of 2050 who had been born in the United States -- 1 million of them with US-born mother and father -- if birthright citizenship were denied to children born to parents who are both unauthorized immigrants. While some policymakers are discussing changes to birthright citizenship as a means to reduce illegal immigration, the report makes clear such a move could in fact significantly increase the size of the unauthorized population.