Women in the U.S. are having fewer children than ever, a December study from the Center for Disease Control and Prevention said.

This is not the first time the CDC released data on low birth rates in the U.S. An earlier CDC report from this year revealed that teenage pregnancy dropped six percent since 2011 and that more women over 30 were having children. But this month's report reveals the U.S. pregnancy rate is the lowest it's been in 12 years.

The CDC study shows that from 2000 to 2009 the pregnancy rate fell by 12 percent, or roughly 6.4 million pregnancies, CNN reported.

The study also found decreased rats across multiple categories. Pregnancy rates for both married and unmarried women have declined close to 10 percent since 1900. The rate of abortions has decreased too, dropping 32 percent between 1900 and 2000, CNN reported.

The December study confirmed previous findings that there are less teenage pregnancies. The teen birth rate in 2009 was 39 percent less than when it peaked in 1991, and has continued to decline up to 2012, the study said.

"Research suggests that more teens are delaying initiating sex, waiting longer to have sex," Rachel Jones, a researcher for Guttmacher Institute, who was not involved in the study, told CNN. "More teens are using more contraceptives and using more effective methods of contraception."

Pregnancy rates for women over 30 have gone up since 1990, with the rate increasing 30 percent for women ages 35 to 39 since 1900, CNN reported.

"The expectation is that women in their 30s have considered career and education, delaying childbirth in their 30s, so they're making a conscientious decision to become pregnant and have a baby," Jones told CNN.

Despite the low birth rates, there is also evidence suggesting that could soon change. CDC's data shows the birth rate from July 2012 to June 2013 was basically the same as the previous year, The Tennessean reported.