"It could be a new trend, but it's just really too soon to say," Stephanie Ventura, who heads the CDC's Reproductive Statistics Branch, said in a telephone interview.
The reasons for the rise in the teenage birth rate are unclear, Ventura said, but she noted there is early evidence that declines in U.S. teen sexual activity have leveled off.
"I don't know if the use of contraception among teens has changed. We just don't know what's happening," Ventura said.
The CDC said the birth rate among the youngest girls - ages 10 to 14 - declined last year.
Births to unmarried girls and women reached their highest levels in 2006 since the government began tracking such statistics in 1940, Ventura said.
Unmarried girls and women accounted for 38.5 percent of all U.S. births last year, up from 36.9 percent in 2005, the CDC said. Among blacks, they accounted for 70.7 percent of births. Among Hispanics, it was 49.9 percent and among whites 26.6 percent, the CDC said.
MORE SOCIAL ACCEPTANCE?
The out-of-wedlock birth rate rose by a sharp 7 percent last year - to 50.6 births per 1,000 unmarried girls and women in 2006 from 47.5 per 1,000 in 2005.
Ventura said that from about 1995 to 2002, statistics on births by unmarried women had remained pretty stable, but since then have been on the way up.
Only about 23 percent of births to unmarried women were to teenagers in 2006. It used to be about half," she said.
Survey data shows about 40 percent of the unmarried women giving birth are in cohabiting relationships, Ventura said.
"There certainly is greater acceptance of children being born out of wedlock. For instance, you see many fewer marriages after conception but before birth than you did in the past," said David Landry, a researcher at the Guttmacher Institute in New York.
The U.S. fertility rate was about 2.1 births per woman of child-bearing age over her lifetime. It is the first time since the early 1970s that the rate was above the replacement level, at which a given generation can replace itself, the CDC said.
The total numbers of births in the United States in 2006 was about 4.3 million, up 3 percent from 2005.
The CDC also said 31.1 percent of all births last year were by Caesarean section, a record high. The proportion of births by Caesarean section has risen 50 percent in the past decade.
"Caesareans are being done too casually and too routinely," Pam Udy, president of the International Cesarean Awareness Network advocacy group, said in a telephone interview.
The CDC said its report was based on data from more than 99 percent of all U.S. births last year.
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