It does not make more sense no matter how I imagine it to refer to women. Once a woman has given birth to a child she is no longer childless. I cannot imagine any way of counting women to arrive at such dramatic changes in numbers of childless women in only three years.
It's a ratio of women who have had children to women that have not had children.
"...it doesn't extend to women who have already had children."
Then it isn't a ratio of women who have had children to women that have not, which is what it is described as.
Something is inexplicable about that number.
You know I've thought about what you wrote and I think you make a good point and I'm confused now as well!