(CNN) -- After two years of increases, the teen birth rate in the United States declined 2 percent between 2007 and 2008, the U.S. Centers for Disease Control and Prevention said Tuesday.
That suggests that the increases in teen births between 2005 and 2007 were a "blip," said Stephanie Ventura, chief of reproductive statistics branch at the National Center for Health Statistics, part of the CDC.
The long-term trend in the teen birth rate has been downward since 1991, Ventura said. But the declines themselves grew more modest in the mid-2000s before turning upward in 2006.
"We could be reaching a place where further decreases are harder to achieve," she said.
A new preliminary analysis found 41.5 births per 1,000 teenagers between the ages of 15 and 19 in 2008. This represents a decrease from 42.5 in 2007 and 41.9 in 2006.
Older teenagers in particular, ages 18 and 19, showed a 4 percent decline in births. The youngest teenagers, however -- ages 10-14 -- saw no change in birth rate from previous years, averaging 0.6 births per 1,000.
Overall U.S. births fell by 2 percent in 2008, the report said. This could be because of the economic downturn, Ventura said, as families may have delayed having children in the face of financial hardship. The previous year saw an all-time high of births in the U.S.: 4,317,119.
The report also showed that women giving birth between ages 40 and 44 increased to the highest rate since 1967: 9.9 births per 1,000 women, up 4 percent since 2007. For women aged 45 to 49, birth rates also went up, from 0.6 births per 1,000 to 0.7 in 2008.
"They may just be reaching what they see as the closing of their biological capabilities," Ventura said of the older women. They may also be using assisted reproductive technologies.
The report also showed that the percentage of babies born preterm, before 37 weeks of pregnancy, went down significantly, from 12.7 in 2007 to 12.3 in 2008.
The rate of low-birthweight babies, weighing less than 5 pounds 8 ounces, went unchanged from 2007, at 8.2 percent. Numbers of low-birthweight babies had been rising since the mid-1980s but began to decline slightly in 2006 and 2007.
There were more women having Caesarean sections in 2008 than the year before, 32.3 percent, representing the 12th consecutive year that this rate went up. This is not fully explained by the rising numbers of older women giving birth, because C-sections went up across all age groups, Ventura said.
Although the birth rate for unmarried women went down 2 percent, the total number of births to unmarried women increased by 1 percent. There were 1,727,950 births to unmarried women in 2008, which is 27 percent higher than in 2002, the report said.