They are similar terms that mean slightly different things.
The birth rate is the number of live births per 1,000 people per year.
The total fertility rate, on the other hand, looks at the average number of children women will have over their lifetimes - how many children a 19-year-old will have, rather than how many she had this year.
Other possible explanations could include changing societal expectations, increased access to contraception, limited provision for parental leave from work, and increased expectations of romantic love.
The rate has been dropped since the post-World War Two "baby boomer" generation was born, and suffered another dip in the recession that started in 2008.
What does it mean for the future?
It could mean there are fewer young people in the workforce in the future, at the same time as life spans increase and more elderly people need care and resources into later life.
But it does not mean the population will shrink. It may grow at a slower rate, but there will continue to be more and more people in the US.
William Frey of the Brookings Institute called for some perspective on the figures.
"The country isn't going to run out of people," he said.