U.S. birth rates in 2009 declined for the second straight year, a sign the economy may be causing some women to think twice about having children, health officials said Friday.

Estimates from the Centers for Disease Control and Prevention show 4,136,000 children were born in 2009, down 2.6 percent from the 2008 estimate.

That followed a similar decline in 2008, the start of the economic downturn that has yet to abate. The CDC said the 2009 numbers are preliminary and could change.

No details are available yet on the characteristics of women who gave birth in 2009.

A preliminary analysis of births in 2008 showed they dropped for women of all ages under 40, but rose for women 40 and over.

That may turn out to be the case in 2009 as well, but we won't know until more data are available, the CDC said in a statement.

The CDC said it is possible the declines are related to the economic downturn in the last couple of years.

The recession appears to have started in late 2007, and declines in births in 2008 and 2009 would be consistent with that, the CDC said.

But health officials will need more details on the demographics of mothers who gave birth in 2009 to confirm the connection.