According the Centers for Disease Control, New York rank number one in the U.S. for women aged 40 to 44 having their first born. New York ranked second for women aged 35 to 39, following Massachusetts.

The number of first time mothers who are aged 35 years and above is nine times more than it was in the 1970s. The increase comes at a time when the number of births in the U.S. is declining every year since 2007.

Women who are between 35 and 39 years old took most of the increase in older first-time mothers. For every 1,000 between 35 and 39 years old women, 11 were first time mothers in 2012. In 1970, there were 1.7 first-time mothers per 1,000 women in this age bracket. In 1985, there were 0.5 first-time mothers in every 1,000 women aged 40 to 44. In 2012, there were 2.3 first-time mothers in 1,000.

Washington D.C. and 46 U.S. states showed an increase of first-time mothers aged 35 to 39 from 2000 to 2012. Washington D.C. and 31 states had an increase of first-time mothers aged 40 to 44. Areas such as Nebraska, Minnesota and South Carolina increased 60 percent or more.

"When childbearing is delayed, it has several ramifications, including smaller-size families and population structure," National Center for Health Statistics and lead author of the study T.J. Mathews said. "There will be many more smaller families if you have your first child after 35, rather than start a family in your 20s the way more women did generations ago."

Asian American women are more likely to be first-time mothers over 35 years old than women with different ethnic and racial backgrounds. Asian or Pacific Islander first-time mothers increased up to 95 percent from 1990. The number of non-Latina black first-time mothers increased up to 171 percent from 1990. The number of non-Latina white first-time mothers increased up to 130 percent from 1990. The number of Latina first-time mothers increased only up to 64 percent since 1990. The number of Alaska Natives or American Indian first-time mothers only increased up to 50 percent from 1990.

Women delay childbirth because they want to achieve educational and professional goals first. The new technology of freezing women's eggs while they are more fertile and younger and even better in vitro fertilization and prenatal care results allow women to have children at their own time.