Monday, December 24, 2012

Science of Vegan Pregnancy 01: maternal serum screening and blood levels of vitamin B12

Nearly ten years ago, a group of Taiwanese researchers came out with a study showing that vegan and vegetarian pregnant women were more likely to receive false positives on a common test called a "maternal serum screening."  This is a big deal for veg*an pregnancies because a positive MSS result can lead to a more invasive procedure, an amniocentesis, that carries a chance of miscarriage and other damage to the fetus.  And of course, a false positive can lead to a lot of unnecessary distress.

In effect, "normal" test results for veg*ans don't look the same as normal test results for omnivores, and that information should affect how doctors interpret our tests.

Maternal serum screening tests for the likelihood that a fetus has Down's syndrome.  The test is a numbers game: it doesn't give you a yes or no answer.  It only determines whether you (the pregnant woman) are more or less likely than other women your age to have a Down's syndrome baby.

MSS measures the levels of several different proteins in a pregnant woman's blood: typically human chorionic gonadotropin (hCG), alpha-fetoprotein (AFP), and unconjugated estriol (uE3), sometimes with the addition of inhibin A.  (FYI, hCG is the same hormone that is detected by pregnancy tests.

If the hormone levels are off (some higher than average, some lower than average), the test will come back positive.  An elevated level of hCG is a risk factor.

Open for interpretation?
[image credit here]
I started looking into this because I'm vegan, and my MSS results came back positive.  That is, I had an elevated chance (compared to other women my age) of having a baby with Down's syndrome.  The previous month, my overworked and barely-English-speaking GP recorded an incorrect date of conception, which meant that I did not receive another common Down's syndrome screening test (a nuchal translucency ultrasound).  Thus, the MSS was the only test we had to go on when deciding whether or not to get an amniocentesis -- and where to go from there.

Enter google scholar, which directed me to this article on MSS and vegetarianism.  The authors of the study are Taiwanese doctors working with Taiwanese women.  Taiwan is a particularly good place to conduct research on vegetarianism and health because: 1. it has a ton of research hospitals; 2. a high proportion of the population is vegetarian for religious reasons -- mostly Mahayana Buddhism, but also various forms of neo-hybrid-Buddhist-Daoism, and completely original religions like that of Supreme Master Ching Hai, who tells all her followers to be vegan.  Taiwan even has a dedicated Buddhist research hospital where all the canteens serve vegetarian food.

We don't all have our own TV stations... [image credit here]
Back to our study.  Cheng et al, the authors of the study, found that:

Vegan and vegetarian pregnant woman have higher average levels of b-hCG.

• This means that midtrimester serum screening produces more false positives for vegan and vegetarian pregnancies.

There's a correlation between lower vitamin B12 levels and higher b-hCG levels.  Thus, women with a lower concentration of vitamin B12 in their blood (but who are not necessarily deficient in vitamin B12) are more likely to get a false positive test result.

• The greater number of false positives on the test leads to more invasive and unnecessary screening procedures (such as amniocentesis) for vegan and vegetarian pregnant women.

And finally...

Low vitamin B12 levels do correlate to at least one other "danger signal" in pregnancy, namely, higher levels of plasma homocystine (associated with a host of nasty problems -- birth defects, pre-eclampsia, etc. etc.).  So in theory, it's possible that low vitamin B12 levels in vegan and vegetarian women would correlate to these kinds of pregnancy complications -- but, as the authors point out, this question needs further research.  (I ain't panicking -- but I am taking B12 supplements, and have been throughout the pregnancy.)

So what happened with my test results?

My GP sent me to an OB/GYN, who told me not to panic.  He arranged a further ultrasound screening that indicated a lower than average likehood of Down's syndrome.  I opted for no amniocentesis.  And that's the end of that story (until the baby's born).

I did tell my OB/GYN about the study, but I live in a town with approximately 2 other vegans, one of which I'm married to... so he may never meet another pregnant vegan in his entire career.

And fortunately, neither my GP (who is a caste vegetarian) nor my OB/GYN (an average beef-eater) ever tried to argue me out of veganism.


Cheng PJ, Chu DC, Chueh HY, See LC, Chang HC, Weng DR.  2004.  "Elevated maternal midtrimester serum free beta-human chorionic gonadotropin levels in vegetarian pregnancies that cause increased false-positive Down syndrome screening results."  American Journal of Obstetrics and Gynecology, Vol. 190 (No. 2), pp. 442-7.