When searching Google, you can limit your search to a US government website by typing in
along with your keywords. An example of a search using this limiter is
site:.gov statistics gum disease
Or any other domain that works for your research, some other examples include:
site:.edu OR site:.org
To include more than one domain limiter use this phrasing where both domains are included in the parentheses and the separator OR is between them, capitalized:
"site:.edu OR site:.gov"