IStock Photo 1820156 © Michael Blackburn

Medical advances over recent decades have redrawn the biological boundary limiting a woman’s ability to conceive and bear children. These advances have come with controversy, including worry about the increased health risks posed by advanced maternal age, and the concern that people who become parents in their sixties and even beyond may not live to raise their children. This debate dates back to at least 1997, when a 63-year-old California woman gave birth following three years of fertility treatments. It has been reignited by more recent cases—including the case of an Indian woman who gave her age as 70 when she delivered a premature set of twins in 2008, and a Spanish woman who gave birth to twins in 2007 and died two years later at the age of 69.

These cases of very advanced maternal age might overshadow a trend affecting a much larger group of women. Middle-aged motherhood has become more mainstream. Over the last 20 years, the number of women having babies at 40 or older has grown four-fold, according to the National Institutes of Health. Some of that surge stems from fertility treatments. More than half of the women who availed themselves of reproductive assistance between 1998 and 2003 were 35 or older.

The odds an older mother will conceive and carry the pregnancy to term have increased dramatically in the decades since techniques like in vitro fertilization have been available. And thousands of women ages 40-44 do plan on becoming pregnant. The odds a woman in that age range intends to have a child in the future are 1 in 22.22. The odds that a woman 40-44 will actually give birth in a year are 1 in 106.6. For a woman aged 45-49, the odds are 1 in 1,777.

Perhaps most important to those over forty planning to become pregnant are the outcomes women their age have once pregnant. The odds the pregnancy of a woman 40 or older will result in live birth are 1 in 1.8 (56%). The odds of an abortion are 1 in 5.21 and the odds of a fetal loss are 1 in 3.96.

While mothers in their 60s make the news and the shift toward more 40-somethings giving birth appears entrenched, the age of first-time mothers actually went down in 2006. According to US Census figures, it's now 25 years old, down from 25.2 years old in 2005. That's the first time the age has dipped since 1968.