The rule of thumb is that about 87% of the women in a population will have children. In the United States and other industrialized nations the percentage has fallen below that as we move from the first wave of Baby Boomers, born in the 1940s, to the first wave of Gen Xers, born in the 1960s. By the time they were 44, the usual end of a woman's fertile years, about 86% of older Boomer women had children. For older Gen Xers, the number is 81%.
Put another way, almost a fifth of older Gen X women, the "Atari wave" as opposed to the "Nintendo wave" of the '70s, will have no children. Gen Xers are more likely to be educated, and to have more education, than their predecessors. All those years of schooling and career starting eat into baby time. This catches many forty-something women by surprise. Also, as the most divorced-upon generation, the Xers were much slower to marry and have kids than their predecessors were, for fear of screwing their kids up.
It is too early to know the completed fertility of the Millennials, or even the later wave of Xers. If the increasing numbers of weddings by young Millennials that I get invited to are any indication, though, the rising generation of fertile women may reverse the trend of their "lost generation" predecessors.