To the Point: Trisomies, such as Down syndrome, are the result of a “chromosome packaging error” in the maturing egg or sperm cell, which is not your fault.
Typically, there is nothing you can do to cause or prevent Down syndrome or other trisomies. At this time, there are no known environmental or behavioral causes; it just happens.
Now that we have that covered, let’s discuss how something like Down syndrome happens. This explanation also applies to trisomies in general such as, Trisomy 18, Trisomy 13, Triple X & Klinefelter syndrome.
As was discussed in Genetics 101, mom’s egg and dad’s sperm each contribute 23 chromosomes to result in the baby having a total of 46 chromosomes or 23 pairs. But, mom’s egg and dad’s sperm don’t start with 23 chromosomes. They start with 46, and it isn’t until they go through a process of “maturing” that they end up with just the 23 chromosomes.
So, picture mom’s egg with the 46 (23 pairs). Prior to ovulation (release of the egg from the ovary), the 23 pairs of chromosomes essentially line up along the middle of the egg and are pulled apart, resulting in 23 of each chromosome in the mature egg. What occasionally can happen is that this process of the pairs being pulled apart does not go according to plan, and the result can be the mature egg ending up with 2 of one particular chromosome instead of one as usual. So, when the sperm enters the picture and contributes its copy of that chromosome, the result is an embryo with 3 copies of one particular chromosome; 2 from mom and 1 from dad. This is a trisomy. This “chromosome packaging error” can happen in the egg or the sperm.