The number of American women without children has risen to an all-time high of 1 in 5, a jump since the 1970s when 1 in 10 women ended their childbearing years without having a baby, according to the Pew Research Center.

About 1.9 million women aged 40-44—or 18 percent—were childless in 2008, an 80 percent increase since 1976, when just 580,000—10 percent of those in that age bracket—had never given birth, the Census Bureau's Current Population Survey shows.

Childlessness has increased across racial and ethnic groups and most education levels, but has dropped among women with advanced degrees in the past decade, according to the research.

Part of the reason for the rise in the number of childless women is an overall pattern toward delaying marriage and having kids, the research showed. Experts say that a drop in societal pressure to be a parent is also responsible for the trend, along with an increase in career opportunities and an improvement in birth control options and effectiveness.

"People put off getting pregnant," said women's health expert Dr. Laura Corio, a member of AOL Health's Medical Advisory Board. "Even when they're married they're putting off getting pregnant. Then they wake up one day and they're 40 and they want to have a child. It's like, hello?"

Explanations for the trend vary, but for some, the reason behind delaying parenting is a simple joie de vivre.

"They're freer. People are enjoying their lives: they're traveling, shopping, eating out," Corio told AOL Health. "Putting a baby into the situation is going to change everything."

Though the most educated women are still among those most likely to never give birth, there was a 31 percent decline in those aged 40-44 with master's, doctoral or professional degrees who had not had babies between 1994 and 2008, when 24 percent of women in that category were childless.

The data represents combined statistics from 2006 and 2008 (referred to as 2008 in the study) and from 1992 and 1994 (referred to as 1994).

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White women remain the most likely not to have had a child, but the childless rates have grown more quickly for blacks, Hispanics and Asians over the past decade, according to the figures.