Why was 35 chosen for advanced maternal age?

One of the most common reasons I meet with a client is because a woman will be 35 or older when she has a baby. This gives a woman the less than sensitive label of “advanced maternal age,” or as some even call it, “elderly maternal age.” These clients make comments sarcastically calling themselves old, and while 35 is far from being old, the medical terminology can feel a bit harsh.
I try my best to normalize being called, “advanced maternal age,” because in 2017, having a baby after 35 is very normal. But socially normal or not, the biological clock doesn’t seem to care. Worldwide, women and couples are waiting longer and longer to have children. But not every woman in the category is there for the same reasons. For some, higher education, career opportunities, and access to effective birth control give the ability (and perhaps the desire) to plan their pregnancies more than ever before. Others may have been open to or even had children earlier in life, but still, find themselves growing their families later than expected.
While there are several different risk factors that increase with maternal age, today I am (shockingly) going to focus on genetics, and the reason why 35 was granted that magical distinction. To begin, we need to take a trip back to high school biology…
 
 

To begin, we need to take a trip back to high school biology…


Each of our cells (with the expectation of eggs and sperm) have 23 pairs of chromosomes, or 46 chromosomes total. We get one in each pair from our mother and the other from our father. Egg cells and sperm cells typically contain just one chromosome from each pair, for a total of 23 single chromosomes. In the developing female fetus, a pre-egg cell (called a primary oocyte) with 46 chromosomes prepares to divide into egg cells with 23 single chromosomes. When a female fetus is about 20 weeks gestation, all of the chromosome pairs prepare for the separation and then freeze just before they divide. This actual division will only happen if the egg cell is ovulated and fertilized. So when a woman gets pregnant at say 39 years of age, that pre-egg cell has been “frozen” for almost 40 years. It’s been holding those chromosome pairs together until that final moment when they can divide. This is a very complex process, but essentially, the longer we ask our pre-egg cells to “hold on tight to your chromosome buddy” so everything can divide properly, the greater the chance that those tight grips will either loosen up or become too tight over time. This increases the chance that the pre-egg will fail to divide evenly (known as nondisjunction). If an embryo ends up with too few chromosomes, it will likely miscarry. If an embryo ends up with too many chromosomes, it will either miscarry or be born with a “chromosome condition,” such as Down syndrome.
Down syndrome, also known as trisomy 21, is a chromosome condition that occurs when a baby has an extra copy of chromosome 21. Down syndrome typically does not run in families, and the risk to have a baby with Down syndrome increases as a woman gets older (for the reason explained in the paragraph above). Many years ago, the only prenatal test for chromosome conditions was an amniocentesis, a procedure in which a needle is inserted into the mother’s abdomen to obtain a sample of the amniotic fluid. This fluid contains the baby’s skin cells that we can use to see the baby’s genetic information. Because this procedure is invasive, it does carry a risk of miscarriage that has historically been reported as 1 in 300. To this day, amniocentesis is still considered the gold standard of prenatal diagnostic testing, but many other ways of evaluating risk for genetic conditions in a non-invasive way now exist.
 
 

Trisomy 21

Amniocentesis

So why is 35 considered advanced maternal age? Because back when an amniocentesis was your only option for prenatal genetic testing, and Down syndrome was one of the only genetic conditions that could be evaluated for, the age of 35 was when the chance for Down syndrome in your pregnancy equaled the risks associated with an amniocentesis. Before 35 you were more likely to have a miscarriage if you did an amniocentesis than be carrying a child with Down syndrome. After 35, you were more likely to learn that the baby had Down syndrome than having a miscarriage because of the procedure. Statistically, it only made sense to offer amniocentesis to women 35 and older. Nothing magic happens to your egg cells on your 35th birthday. 35 was just chosen because that was where 2 curves on a graph crossed.
 

From the National Down Syndrome Society (http://www.ndss.org/Down-Syndrome/What-Is-Down-Syndrome/)

***Of Note: Every maternal age chart is slightly different, typically depending on if they are giving risks at conception, mid-trimester, or at delivery. Don’t worry that this particular chart shows 1 in 300 risk is associated with being 36…. just focus on the trend 🙂

The best way to feel confident about your family planning and about the decisions you make is to ask questions and be informed. Every pregnancy has risks, everything we ever do has risks. Understand your personal situation and your options for getting more information. And if you ever want a friendly genetic counselor to lend a hand, just let us know!

 

-Erin
 
Erin O’Toole, MS, CGC
Owner, Genetic Counselor
Family Forecast

One Response

  1. Very informative and digestible!

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