During those 20 years, 80.83 million babies were born in the United States, and slightly more than 29 million of them were born to single women, according to the Centers for Disease Control and Prevention.
And in each of the last six years — 2008 through 2013 — at least 40 percent of American babies were born to unmarried mothers, the CDC noted.
In 1940, only 3.8 percent of American babies were born to unmarried women, and by 1960 the figure was still just 5.3 percent.
But the percentages began to climb significantly in the mid-1960s, reaching 10 percent in 1969, 20 percent in 1983, and 30 percent in 1992.
The burgeoning rate of women giving birth outside marriage has economic repercussions. In 2012, the poverty rate for single-mother families was 40.9 percent, compared to 8.9 percent for married-couple families, according to the National Women's Law Center.
And 80 percent of homeless families are headed by single women with children.