The World Bank estimate of 2018 puts the Puerto Rico which is ranked separately from the U.S. in 199th place out of 185 sovereign states and 15 separately listed dependencies in total fertility rate (approximately total number of children born per lifetime at current birth rates adjusted for age) with a TFR of 1.0.
Why is Puerto Rico at the bottom of this list? (The remainder of the U.S. is in 145th place and significantly higher.)
It isn't nearly as urban as countries like South Korea and Japan with very low TFR, and its per capita income is much lower than the rest of the U.S. which is usually associated with higher TFR.