the lowest reported in two decades. The teen birth rate had been declining from 1991 through 2005, but rose from 2005 to 2007. The new data indicate the spike has ended.

Even so, women in their 40s still have babies far less often than younger women. The rate was about 10 births per 1,000 women in their early 40s, and less then 1 per 1,000 for women in their late 40s.

The rate for women in their late 20s was 115 per 1,000.

Experts note that the postponement theory may explain why younger women had lower birth rates, but doesn’t fully get at why the rate for women in their early 40s rose so sharply.

Some speculated that more sophisticated assisted reproduction services may be paying off for older couples, or perhapssome divorced women are choosing to have additional children with a new partner later in life.

The new CDC report also showed that the percentage of babies born prematurely fell a bit, from 12.7 percent to 12.3 percent, an improvement celebrated by some health advocates.

From 1990 to 2006, the nation saw a 20 percent rise in the rate of premature births – a worrisome trend because preemies are more fragile. Experts believe premature births are the main reason the U.S. infant mortality rate is higher than in most European countries.

The 2008 decline may be a sign of new efforts by doctors and mothers to bring births to full term and, when possible, to have one baby at a time rather than twins, triplets or other multiples. Multiple births generally