While it has been known for a long time that the risk or chance to have a baby with Down syndrome goes up as a woman gets older, the exact reason for this increased risk is unknown. It is speculated that the increased risk is due to the fact that a woman’s eggs are as old as she is. Every woman is born with all of the eggs she ovulates for the rest of her life. So if you're 30 when you conceive a pregnancy, then the egg that you conceived the pregnancy with is also 30 years old. If you are 45 when you conceive a pregnancy, then the egg that you conceived the pregnancy with is 45 years old. It is thought that as eggs age, they are more prone to the errors that result in trisomies, including trisomy 21 (Down syndrome). To put it simply, the older your eggs get, the more prone they are to make mistakes.
Why doesn’t a father’s age count?
Basically because men make sperm continuously throughout their life. So a man may be 45, but the sperm that conceives a pregnancy may only be a few weeks old. This seems to provide a protective effect against the type of chromosomal error that lead to Down syndrome and other trisomies. While a father's age doesn’t contribute to the risk for chromosome abnormalities, they are not completely off the hook. An advanced paternal age will influence the chances of other genetic diseases, achondroplasia (dwarfism), Marfan syndrome (a genetic syndrome characterized by tall stature and heart problems) and other autosomal dominant disorders, although this risk is small.
What is “advanced maternal age”?
“Advanced maternal age” is a medical term that is used to describe pregnant women over the age of 35 years which used to be relatively rare. Today, it is not uncommon for older or “advanced maternal age” women to get pregnant and this term is slowly falling out of use. While being of “advanced maternal age” has implications for your chance to have a baby with Down syndrome, you should also consider other health implications.
Who Should have Prenatal Testing?
Prior to 2007, the American College of Obstetrics and Gynecologists (ACOG) used to recommend that all women over the age of 35 be offered prenatal diagnostic testing such as amniocentesis and chorionic villi sampling. There was nothing magic about the age of 35 -- your risk didn’t suddenly shoot higher -- it was just that at the age of 35, your risk to have a baby with a chromosome problem was about 1 in 200 (or ½ of 1%) and the risk to have a miscarriage from an amniocentesis was thought to be about 1 in 200 (or ½ of 1%). So age 35 was picked as the “cutoff” for diagnostic testing because the chance to find a problem was the same as the chance to cause a problem.
In 2007, ACOG changed its guideline on which pregnant women should be offered prenatal testing. Two factors influenced this change in policy -- one was that it was realized that the risk to cause a miscarriage by having an amniocentesis is lower than 1/200 and is probably closer to 1/500. The second is that there were many medical malpractice lawsuits against obstetricians in cases where younger women had babies with Down syndrome but weren’t offered prenatal testing.
It is important to note that the ACOG guidelines simply talk about who should be offered prenatal testing. They do not recommend that all women have testing, just that all women be offered testing. It is still up to each woman to decide if she wants prenatal testing and what type of testing, if any, is right for her.
Making Your Decision and Knowing Your Risks
While all pregnant women are now supposed to be offered prenatal testing, it is still up to each women to decide if this testing is appropriate for her. There are many factors to take into account when making a decision about prenatal testing, but one factor that is important to understand thoroughly is your risk to have a baby with a chromosome abnormality.
Newberger, D., Down Syndrome: Prenatal Risk Assessment and Diagnosis. American Family Physician. 2001.
American College of Obstetricians and Gynecologists (ACOG). Your Pregnancy and Birth, 4th Edition. ACOG, Washington, DC, 2005.
Hook EB, Cross PK, Schreinemachers DM. Chromosomal abnormality rates at amniocentesis and in live-born infants. JAMA 1983;249(15):2034-38.