LOS ANGELES - Births to women ages 15-19 and 20-24 in the United States declined to "historic lows" in 2011, declining to 31.3 births per 1,000 women, said researchers at the U.S. Centers for Disease Control's National Center for Health Statistics.

Writing in the journal Pediatrics on Monday, Brady E. Hamilton and colleagues summarized vital statistics from birth certificates and death records. Overall, there were 3,953,593 births in the United States in 2011, 1 percent fewer than in 2010. The birth rate was 12.7 births per 1,000 total population, the lowest rate ever reported in the nation. Breaking the data down by ages, birth rates fell for women ages 15-29 and rose for women 35-39 and 40-44. Rates were unchanged for women 30-34 and 45-49.

Significantly, among teenagers 15-19 the birth rate fell 8 percent. The authors wrote that the overall teen birth rate fell 49 percent from 1991 through 2011, creating a "substantial" impact.

If 1991 birth trends had persisted, they said, there would have been an additional 3.6 million births to 15- to 19-year-olds.

The analysis also included data on births broken down by marital status -- in all, 40.7 percent of babies were born to unmarried women in 2011, a slight decline from 2010 -- and by race. The rate of Cesarean delivery was unchanged at 32.8 percent. The twin birth rate fell slightly, to 33.1 twins per 1,000 births, and triplet births also fell.

The report also summarized mortality data: overall, 2,513,171 people died in the U.S. in 2011, at a rate of 7.4 people per 1,000 -- a record low.

Among the deaths, 23,910 were infants and 20,192 were children 1 to 19 years old. The leading cause of death in kids was accidents (35.6 percent of deaths) and the second leading cause was homicide (11.4 percent of deaths).