According to the Associated Press, the weak economy may be a decisive factor:
Experts believe the downward trend is tied to the economy, which officially was in a recession from December 2007 until June 2009 and is still flagging. The theory is that women who are unemployed or have other money problems feel they can't afford to start a family or add to it.
In 2008 and 2009, the only increase in births was in women older than 40 — considered more sensitive to the ticking of their biological clocks.
A drop in immigration to the United States, blamed on the weak job market, may be another factor in last year's decline.