The U.S. says its birth rate has dropped for the third straight year, with the birth rate for the nation's youngest mothers reaching an all-time low.

U.S. health officials said Thursday that just over four million babies were born in the country in 2010, down from the all-time high of more than 4.3 million in 2007. They said the U.S. birth rate is falling for most all age groups of women and all races.

The drop in the birth rate was most pronounced for teenagers, down 9 percent to about 34 births per 1,000 women. The birth rate for women in their early 20s was off 6 percent. Together, the birth rate for the youngest mothers was the lowest since records were first kept in the 1940s. The birth rate increased only for women in their early 40s, up slightly to about 10 births per 1,000 women.

One advocate for the prevention of teenage pregnancy, Sarah Brown , said the drop in teen births is part of a 20-year trend during which the teen birth rate has dropped 44 percent. She attributed it to teenagers having less sex and the greater use of contraception.

Population experts said they believe the decline in the overall birth rate in recent years, particularly among young women, is attributed to the nation's sluggish economy. They said many couples are worried about losing their jobs and whether they have enough money to start a family or add to it.