 The average annual net migration rate of Mexico City over the same period was negative and the lowest of all political divisions of Mexico, whereas the states with the highest net migration rate were Quintana Roo (2.7), Baja California (1.8) and Baja California Sur (1.6).While the national annual growth rate was still positive (1.0%) in the early years of the 2000s, the national net migration rate was negative (-4.75/1000 inhabitants), given the former strong flow of immigrants to the United States; an estimated 5.3 million undocumented Mexican immigrants lived in the United States in 2004 and 18.2 million American citizens in the 2000 Census declared having Mexican ancestry.However, as of recent years in the 2010s, the net migration rate reached 0, given the strong economy of Mexico, changes in US Immigration Policy & Enforcement, US Legislative and CFR-8 decisions, plus the (then) slowly recovering US economy, causing many of its former residents to return.