With Comments from Russell H. Greenfield, MD
Vegetarian Diet and Screening for Down Syndrome
Source: Cheng P-J, et al. Elevated maternal midtrimester serum free b-human chorionic gonadotropin levels in vegetarian pregnancies that cause increased false-positive Down syndrome screening results. Am J Obstet Gynecol 2004;190:442-447.
Goal: To determine the effect of a vegetarian diet on maternal serum levels of free b-human chorionic gonadotropin (b-hCG) and alpha-fetoprotein (AFP) for Down syndrome screening.
Subjects: This study examined 98 lactovegetarian and 122 omnivore Taiwanese women with singleton pregnancies, and without maternal complication or abnormal outcomes.
Methods: Screening for Down syndrome was performed between 14 and 18 weeks of gestation via enzyme or radioimmunoassay. Results of free b-hCG and AFP levels were compared between groups and with reference levels. Serum B12 concentrations were also determined.
Results: Levels of free b-hCG were significantly higher in the lactovegetarian group as compared with reference levels. This resulted in a false-positive rate for the presence of Down syndrome of 17.3%. The vegetarian group also had a significantly lower mean serum vitamin B12 concentration. On further analysis, levels of free b-hCG were highest in the vegetarian pregnancies that also had the lowest serum levels of vitamin B12. Those vegetarian pregnancies with normal B12 levels had free b-hCG levels comparable to those of the reference population. No differences were noted between groups with respect to AFP levels.
Conclusion: The false-positive rate with midtrimester screening for Down syndrome is higher than normal among vegetarian women with low serum levels of vitamin B12. New reference levels for free serum b-hCG for vegetarian women should be established to correct this situation.
Study strengths: Sample size; degree of follow-up.
Study weaknesses: Inadequate discussion of sample size determination, including lack of exclusion criteria; no mention as to whether any subjects had access to prenatal vitamins.
Of note: A lactovegetarian was defined in this study as someone who eats eggs and dairy products, but no meat, fish, or poultry; the false-positive rates for the reference and omnivore groups were 5.3% and 5.7%, respectively; the authors recommend screening for Down syndrome using ultrasound markers and nuchal translucency measurement, rather than serum markers, for vegetarian women.
We knew that: Numerous other factors influence the accuracy of screening for Down syndrome, including maternal weight, smoking, and the presence of illnesses such as SLE and IDDM; most maternal serum AFP is of fetal origin, whereas hCG is of placental origin; adequate vitamin B12 is necessary for proper DNA synthesis.
Clinical import: The importance of folic acid supplementation during pregnancy has been recognized for some time, and in this regard public health initiatives have helped to lower the incidence of birth defects. It also has long been known that people adhering to a strict vegetarian/vegan diet often are deficient in specific nutrients, including zinc, calcium, iron, and vitamin B12. It now appears that inadequate levels of vitamin B12 during pregnancy can complicate screening for Down syndrome, leading to unnecessary interventions. While the authors call for the development of new serum b-hCG reference levels for vegetarian women, the results also can be taken as further call to ensure that all pregnant women have access to both nutritious foods and prenatal vitamins.
What to do with this article: Keep a copy on your computer.
Dr. Greenfield, Medical Director, Carolinas Integrative Health Carolinas HealthCare System Charlotte, NC, is Executive Editor of Alternative Medicine Alert.