The new report also reveals some major state-by-state differences in fertility rates.
Overall, the total fertility rate for the United States in 2017 was 1,765.5 per 1,000 women, which was 16% below what is considered the level needed for a population to replace itself: 2,100 births per 1,000 women, according to the report.
The researchers found that South Dakota, with a rate of 2,227.5, and Utah, with a rate of 2,120.5, were the only states with a total fertility rate above replacement level in 2017.
The report also showed differences in total fertility rates by race: Among non-Hispanic white women, no states had a fertility rate above the replacement level; among non-Hispanic black women, 12 states did; and among Hispanic women, 29 states did.
For non-Hispanic white women, the highest total fertility rate was in Utah, at 2,099.5, and the lowest in the District of Columbia, at 1,012.
Among non-Hispanic black women, the highest total fertility rate was in Maine, at 4,003.5, and the lowest in Wyoming, at 1,146.
For Hispanic women, the highest total fertility rate was in Alabama, at 3,085, and the lowest in Vermont, at 1,200.5, and Maine, at 1,281.5.
The report had some limitations, including that for some groups of women, the number of births used as the basis for calculating total fertility rate was small.
Yet, in general, "although nearly all states lack a (total fertility rate) that indicates their total population will increase due to births, these results demonstrate that there is variation in fertility patterns within states among groups according to race and Hispanic origin," the researchers wrote in the report.
Previous research has shown that US birthrates appeared to hit a "record low" in 2017, when the number of births nationwide was at its lowest in three decades.
"We have been seeing fertility rates go down, and I think it has a lot to do with women and men, couples in particular, having much more control over their reproductive lives," Benjamin said.
When considering rates over larger periods of time, "remember that we're coming off of a peak of the baby boom generation. So it's also being tracked from that very high baby boom that we had after World War II, and so you're really looking at reductions from that high," Benjamin said.
"I think the concern is -- and there is a concern -- is having a fertility rate that doesn't allow us in effect to perpetuate our society," he said. "But we may very well over time start seeing this reversed or flattened out, but that remains to be seen."