The number of babies being born in the United States has reached its lowest level in 100 years.

The U.S. birth rate has dropped for the second year in a row.

Just over 4 million babies were born last year -- nearly a 3 percent drop from 2008.

"It's a good-sized decline for one year. Every month is showing a decline from the year before," said Stephanie Ventura, the demographer who oversaw the report.

The birth rate -- which takes into account changes in the population -- fell to 13.5 births for every 1,000 people in 2009. The numbers are down from 14.3 in 2007 and way down from 30 in 1909, when it was common for people to have big families.

Experts say today, more couples are putting off having children because of the economy.

They also point to a decline in immigration.