Complex sentence: The growing presence of first generation immigrants in the US has led to a growth in the percentage of the population that can be categorized as second generation immigrants, as immigrants are more likely than native born adults to have children.
Split into simpler sentences: The growing presence of first generation immigrants in the U.S. has led to a growth in the percentage of the population that can be categorized as second generation immigrants. This is due to immigrants being more likely than native born adults to have children.