Folate and vitamin B12 levels in early pregnancy and maternal obesity

There is good evidence that periconceptual Folic Acid (FA) supplementation can prevent two thirds of Neural Tube Defects (NTDs). A two-fold increase in NTD rates have been associated with maternal obesity and, based on limited evidence, national guidelines have recommended prescribing high dose FA for women with a Body Mass Index (BMI) >29.9 kg/m². This observational study examined the relationship between maternal BMI and serum folate, red blood cell (RBC) folate and plasma vitamin B12 measurements in early pregnancy.

Women were recruited at their convenience during their first antenatal visit to the hospital following sonographic confirmation of an ongoing pregnancy. Clinical, sociodemographic, dietary and supplementation details were collected and computerised. At the time of routine phlebotomy, samples were collected for serum folate, red blood cell (RBC) folate and plasma B12.

Of the 496 women, 19.6%. (n = 97) were obese based on a BMI > 29.9 kg/m². After excluding energy under-reporters, there was no difference between obese women and women with a normal BMI in their dietary or supplementation intakes of folate. Compared with women with a normal BMI (n = 263), obese women had a lower median serum folate (32.0 nmol/L IQR 20.2 vs 36.2 nmol/L IQR 16.3, P = 0.02) and a lower median serum B12 (203.0 pmol/L IQR 102.5 vs 208.0 pmol/L IQR 125.3, P = 0.03), but there was no difference in the mean red blood cell (RBC) folate measurement. There was a negative correlation between increasing BMI and both serum folate (P = 0.03) and plasma B12 (P = 0.03), but no correlation between BMI and RBC folate (P = 0.13).

Our findings support existing recommendations that obese women should be prescribed higher doses of FA periconceptually. However, to prevent NTDs successfully they may also require B12 supplementation.