That is the principal finding of a study by the Graduate School of Public Health at the University of Puerto Rico, based on an analysis of birth certificates issued from 1990-2004.
A sampling of data from 2007 shows no change in the percentage, the school's dean, Dr. Jose Cordero, told Efe Monday, adding that the World Health Organization estimates Puerto Rico's rate of pre-term birth may be the highest in the world.
Worldwide, about 13 million babies are born premature every year, according to the WHO.
The rate of pre-term births ranges from 3.8 percent in Central Asia to 17.5 percent in southern Africa.
Puerto Rico's 20 percent rate compares with 12.3 percent for the United States as a whole.
Factors contributing to the rate of pre-term births in Puerto Rico include smoking, the high incidence of diabetes among expectant mothers and a lack of pre-natal care, Dr. Cordero said.
The study, directed by Cordero and demographer Hernando Mattei, also showed a decline in annual births in Puerto Rico, from 60,000 in 1990 to 45,000 in 2004.
Cordero suggests births are down because of wider use of contraception and the aging of the Puerto Rican population.
Mattei and Cordero are currently examining the possibility that factors such as pollution, the quality and availability of healthcare or factors specific to the lives of mothers in Puerto Rico have a bearing on the rate of premature births.