One in five women in America are remaining childless throughout their lives, twice the proportion of a generation ago.

A new survey from the US Census Bureau shows that 20% of women aged 40 to 44 were childless, up from 10% in 1976. Though the report - which is based on figures for 2006, the most recent year collected - stops at 44, the number of women giving birth after that age is statistically nominal.

The trend towards having children later in life and the growing proportion of childless women is in tune with patterns across developed countries. Fertility experts have been struck, though, by the steady decline in fertility in the US since the mid-1980s.

"The proportion of childless women has been increasing steadily by about one percentage point a year," said Jane Dye, the report's author.

The bureau's statistics give no clue as to whether US women are consciously putting their careers before their families and remaining childless, or are experiencing fertility problems perhaps as a result of trying to become pregnant towards the end of their fertile years.

Dye pointed out that the figures do give a clear indication of women's decision to have children later in life to allow them to complete their education and establish a career.

The tables show a spike in fertility among women with at least a bachelor's degree in the 30 to 34 age bracket.

"Women are waiting longer to finish their education so childbearing starts later," said Jennifer Manlove, a researcher specialising in teenage pregnancies with the organisation Child Trends.

Not only are highly educated women delaying pregnancy, but they are also more likely to remain childless. The proportion of graduates or those with a professional degree without children aged 40 to 44 rises to more than one in four - 27%.

That compares with just 15% - less than one in six - for women who did not complete their schooling.

The findings also highlight the shrinking of the average American family. In 1976, women on average had 3.1 children, but that figure had fallen by 2006 to 1.9 children. That is below the level of fertility needed to ensure a stable population - 2.1 children per woman is known in demographic jargon as "replacement-level fertility".

Expressed another way, 30 years ago 59% of women used to have three or more children, but that has also tailed off to just 28%.

There is also a striking variation in the frequency of childless women according to their race or origin. The proportion of white women, excluding those of Hispanic origin, who have no children is much higher than other groups at 23%.

Hispanic women are the least likely to remain childless at 14%, with black women at 16% and Asian women at 18%.

The report marks a huge stride in statistical understanding of fertility issues. For the first time the data has been drawn from a massive sample of 3m addresses across the US in the American Community Survey, allowing analysis of fertility annually and state by state.

The findings reveal, for instance, that Utah, Nebraska and Idaho in the middle and west of the US have the highest fertility levels while the north-east states of New Hampshire, Vermont and Rhode Island have the lowest.