The bureau said an estimated 4.1 million women between the ages of 15 and 50 gave birth during the past year and 53 percent of women between the ages of 15 and 44 were mothers in 2010.
The data, released to mark the upcoming Mother's Day holiday, said 81 percent of women aged 40 to 44 in 2010 had given birth at least once, compared to 90 percent for the same age group in 1976.
The average age of women when they gave birth for the first time was 25.4 in 2010, up from 25.2 in 2009.
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