Answer by Steven Fowkes:

Yes, Down's syndrome can be prevented. The chromosomal aberration is caused by a spindle malfunction, which normally pulls each duplicated chromosome pair apart to give each daughter cell one copy of each duplicated chromosome. But with the malfunction, one daughter cell gets both chromosomal copies. This is partly influenced by genetic methylation processes that depend on folic acid and the enzymes that use folic acid. Folic acid can be supplemented, and is routinely supplemented in many food fortification programs.

This is not an uncommon process. Many "healthy" individuals have trisomic cells floating in their blood streams. Possibly, most (!) cytotoxic chemotherapy patients have (temporarily) large numbers of trisomy 21 cells in their blood, particularly when they use folate antagonists as part of the chemo cocktail.

When the spindle malfunction happens, 99% of the time the affected cells are not viable. They die. But trisomy 21 cells are viable; the 21st chromosome is rather small and has less catastrophic influence.

When the spindle malfunction affects only the 21st chromosome, the one-chromosome daughter cell dies and the three-chromosome daughter cell survives. When this happens in a newly fertilized egg, all the subsequent daughter cells are trisomy-21 and this causes classic Down's syndrome. When the spindle malfunction happens during fetal development, some cell lineages are trisomic and some are not. This is called mosaic Down's syndrome.

The likelihood of Down's syndrome increases with age because methylation trends downward with aging, like most aspects of homeostatic regulation. Basal metabolism goes down, protein synthesis goes down, immune functionality goes down, etc. These trends are small, but cumulative.

But there is another risk factor that ties directly with methylation: genetic variations (polymorphisms) of the MTHFR gene. Methylene tetrahydrofolate reductase is one of the enzymes that uses folic acid to accomplish the methylation (regulation) of genes. In climates where folic acid is hard to find some of the time (i.e., northern latitudes), MTHFR mutations have been conserved. There are seven MTHFR mutations that have some degree of frequency, and two that are relatively common (roughly 30% of the general population, and above 50% in Norwegians, Swedes, Finns and other peoples who live close to the Arctic circle. The a1298c and c677t polymorphisms increase the risk health of a variety of health problems, including Down's syndrome pregnancies, and decrease the risk of overall mortality during winter months only if and when green leafy plant foods are in limited supply.

Supplementing various forms of folic acid minimize these risks. But it is not as simple as taking just any folic acid supplement. There are seven kinds of folate in the body and most have vital rolls to play in health and wellbeing. When one folate pathway is obstructed, folate trapping results, which causes one form of folate to become too abundant and another form or forms to become deficient. If you'd like more information on this, I have a free download of  85 pages on Down's syndrome treatment and prevention on the Steve page at Project Wellbeing that were published by CERI in the 90s.

The status of your methylation reactions in your body can be assessed with a variety of techniques. On a standard blood test, the homocysteine level provides some indication. A urine organic acids profile gives more detailed information, but requires greater knowledge for interpretation.

The genetic side, MTHFR mutations, can easily be assessed by a $99 genetic screening from 23andme, or some other company.

The MTHFR polymorphisms tend to be associated with greater risks relating to Down's syndrome births, miscarriages, twinning and neural-tube birth defects. If those run in your family tree, get tested.

There are many over-the-counter supplements that carry methyl groups. TMG (betaine free base) and choline are two well known ones.

I believe that metabolic treatment begun in the first months after birth can prevent the majority of mental retardation, and that at any age, much of the immune dysfunction related to upper respiratory infections and hospitalizations can be prevented. This is all detailed in the above links and in other DS questions I have answered on Quora.

What causes down syndrome? Can it be prevented?