Tuesday, February 19, 2013

Contrary to popular belief: Births to older mothers not at historic high

Sometimes "common knowledge" is not knowledge at all...

There has been considerable hand-wringing in the press over births to women over the age of 35. According to the Telegraph, older mothers are "driving up the birth defect rate." The New Republic suggests that older parents will "upend American society." And Slate, generally progressive, bemoans the "feminist fertility myth."

As a demographer, I am aware that birth rates in age groups 35-39, 40-44, and 45+ have been rising since the late 1970s for ages 35-39 and 45+ and since the early 1980s for the 40-44 set.

But as the granddaughter of a woman who birthed her youngest child when she was nearly forty, my skeptic antennae went up. My grandmother may have been a singular woman, but she was hardly a statistical aberration.

As a historical demographer, who has some experience with fertility and mortality rate trends over the past century (and the century before), I suspected the evidence of a "major shift" might be lacking, and I set out to prove it.

While the fertility trend of delayed childbearing and concomitant increase in birth rate for mothers age 35 and older is occurring in nations around the world, I will focus on data from the United States.

The National Vital Statistics Reports, published by the U.S. Centers for Disease Control and Prevention, provide historical birth rate data by age of mother as far back as 1970. Earlier data must be compiled from a variety of other sources including the older, and often PDF-scan-only Monthly Vital Statistics Reports and the U.S. Statistical Abstract.

From those sources I collected data as far back as 1920, with complete annual data from 1935-2010 (the most recent year for which complete data are available). The historical birth rates (births per 1,000 women) are shown in the chart below.
Sources: NVSR, MVSR, Statistical Abstract, and author's calculations

Zeroing in on ages 35 and older, to show more detail on the chart scale, we see that birth rates are indeed higher in 2010 than they were in 1970. Specifically, birth rates are 45% higher for ages 35-39, 26% higher for 40-44, and 40% higher for ages 45-49 in 2010 than in 1970.

Sources: NVSR, MVSR, Statistical Abstract, and author's calculations
But we do not need to go back very much farther to find a time at which birth rates were higher than they are today: In 1965 birth rates were higher for all ages 35-49 than they were in 2010.

If we go as far back as 1920, birth rates were nearly twice as high for ages 35-39, were three times higher for 40-44 year olds, and were more than 5 times higher for ages 45-49 than the rates in 2010.

In short, the phenomenon of increasing birth rates for women in their late 30s and 40s is as much a return to historical patterns as a deviation from them.

To be clear, some pregnancy risks do increase with advanced maternal age:
"older age strongly increases a woman's chances of at least three untoward outcomes—namely, stillbirth, miscarriage, and ectopic pregnancy" (Stein and Susser 2000)
However, there is considerable evidence that socio-economic factors play the most important role in birth outcome - perhaps far outweighing the age effects (Cohen 2012, Azimi and Lotfi 2011, Stein and Susser 2000).

As for the current focus on trends in the past four decades, I suspect the ease of collecting data from 1970-present is partly the cause of the focus on a "huge" increase in childbearing at older ages... when, in reality, rates have fallen if you look back just 5 years more.

I am happy to share the raw data upon request. Feel free to contact me for more information.


  1. Nice work, Beth. I am fond of taking the long view also. It is personal for me, since I was born in 1962 to a 38 year old mom who had her first child at age 22. I've always been intrigued by how many of my friends born around 1960-1962 have parents from the "G.I. Generation" and siblings 10-15 years older. That is an important part of what characterizes the baby boom, beyond the sheer numbers.

  2. The absolute birth rates you use are confounded by the overall decreasing birth rates. Working with percentages in each age category would be much more informative. Now ages 30-34 (slightly elevated risk of birth defects) have a greater portion of births than ages 20-24 (lowest risk of birth defects). Back in 1960 there were over 60% more mothers in the lower risk category than the higher one.

    In 1960 there were about 7.5 times as many mothers in their 20s as in the highly elevated risk group age 35-39, now the ratio has fallen to about 4.

    In 1960 there were about 28 mothers in their 20s for each mother age 40-45; now the ratio is about 20. To put this in context, this high age group is about as likely to have birth defects as a 20-year old having a child by her half-brother. (Estimated from cousin vs. unrelated partner rates and genetic theory.) Considering only Down's syndrome, the risk in the higher age group is at least 40 times greater than in the lower age group.


your insights?