Maternal vitamin B12 and small-for-gestational age risk after metabolic bariatric surgery: an observational cohort study

Background During pregnancy after metabolic bariatric surgery (MBS), there is an increased risk of small-for-gestational age (SGA) infants. Low maternal vitamin B12 has been suggested as a contributing factor, with a potential modifying role of folate status. Objective To investigate the association between maternal vitamin B12 status and risk of SGA in pregnant women with a history of MBS. Setting Three general hospitals; The Netherlands. Methods Prospective, observational cohort study(n=125). Routine blood samples including plasma vitamin B12 and serum folate were analyzed every trimester (T1;T2;T3). Associations between SGA and plasma vitamin B12 status were analyzed using Linear mixed models and Cox proportional hazard regression models. Effect modification by serum folate was tested by adding interaction terms to the respective models. Results Mean plasma vitamin B12 levels significantly decreased during pregnancy, with plasma vitamin B12 levels being lower in the SGA group (n=19, 15%) compared to the non-SGA group (247 pmol/L [155-338 pmol/L] vs 352 pmol/L [303-400 pmol/L], p =.02). In T1 and T2, women with high plasma vitamin B12 levels had a 70-77% lower risk of SGA compared to women with low vitamin B12 levels (T1: adjusted RR 0.23 [0.07-0.70]; T2: adjusted RR 0.30, [0.10-0.92]). In T3, high vitamin B12 was only associated with a reduced risk of SGA among women with high folate levels (adjusted RR 0.22 [0.05-1.03]). Conclusions This study indicates an inverse association between maternal vitamin B12 status and risk of SGA in pregnant women with a history of MBS, especially in case of high folate status.