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One such situation is if the mother has certain mutations in the MTHFR gene. People with this mutation need extra folic acid. Something called homocysteine builds up in their blood and they need folic acid to combat its effects.

One of the effects of increased homocysteine build up may be more lost pregnancies. One number I saw was that 21% of women with high homocysteine levels have recurrent pregnancy loss! But if this is true (and I haven't seen the hard data to confirm it), is it because of chromosomal abnormalities? Or something else?

Hard to say. There are a couple of ideas out there for how high homocysteine levels might affect a pregnancy. There is some evidence that clots form more easily in these women’s blood. Too much clotting can lead to problems like cystic hygromas which are known to affect pregnancies.

Another possible consequence of high homocysteine levels is chromosomal problems. DNA is often thought of as unchanging but it is actually pretty dynamic. While the A, G, C, and T’s rarely change, something called DNA methylation can change a lot.

DNA is decorated with little chemical groups called methyls. These methyl groups can affect whether a gene is turned on or off. They can also affect whether a chromosome goes to the right place when a cell divides.

The connection here is that high homocysteine levels decrease your pool of methyl groups. What this means is that there is less methyl around so your DNA might not get enough. What folic acid does is provide extra methyl groups to the pool.

It certainly looks like women with the MTHFR mutation have more babies with Down syndrome when they don’t get extra folic acid. But, is there a connection with other chromosomal problems?

No one knows for sure. In one study, researchers failed to find a connection between the MTHFR mutation you mention and any other trisomies except, possibly trisomy 18. However, the researchers couldn’t tell from that study whether the mothers had had enough extra folic acid in their diet to overcome their mutation.

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