Fig 4 Using model developed with Chinese data to predict prevalence of neural tube defects from population level red blood cell (RBC) folate concentrations in US population before and after mandatory folic acid fortification. *RBC folate concentrations from Pfeiffer et al³³ (supplementary table D, female participants 4 years and older during pre-fortification period, NHANES 1988-94; supplementary table F, total female participants aged 4 years and older, NHANES 2005-10). Published RBC folate concentration centiles were normalized to method used in papers by both Daly and Hao, using standardizing equations generated from Pfeiffer 2011³⁵: NHANES RBC folate (nmol/L)=(Dublin RBC folate (nmol/L)×0.7876)+34.2802 (nmol/L) (personal communication). These adjusted RBC folate concentrations were used to generate a modeled population with a similar distribution of RBC folate concentration; modeled associations in Chinese data were then used to predict risk in that modeled US population. †Neural tube defect prevalence for US (1994-95) just before start of mandatory folic acid fortification.³⁹ ‡Neural tube defect prevalence for US (2004-06) after implementation of mandatory folic acid fortification⁴⁰