title: Method of weighted proportion of reproductive-aged women taking folic acid supplements to predict a neural tube defect rate decline.
abstract: BACKGROUND
  Neural tube defect (NTD) rates can be lowered by increased consumption of folic acid (FA) by women before and during early pregnancy. The crude proportion of reproductive-aged women taking FA supplements has been used to predict a decline of the NTD rate in the general population. In this study we examine the potential error in using the crude proportion to predict NTD risk reduction, and offer an alternative method.
  
  
  METHODS
  The crude proportion measures the number of women taking FA. It ignores the substantial variability by maternal age in the probability of giving birth. Age-specific fertility rates (ASFRs) reflect the probability that a woman in a specific age group will give birth in a given year. In this study, we show how to calculate a proportion weighted by ASFRs to predict a decline in the NTD rate, and to assess the effectiveness of FA consumption in preventing NTDs.
  
  
  RESULTS
  Our results show that a crude proportion of 50% of women (15-49 years old) taking FA is associated with a range of 24-77% in weighted proportions. Assuming a 40% risk reduction from taking 400 microg of FA daily, the expected NTD rate decline could vary from 9.6% to 30.6%, depending on the age distribution of women taking FA.
  
  
  CONCLUSIONS
  The ASFR-weighted proportion estimates the proportion of babies born to women taking FA, as opposed to the crude proportion of women taking FA. We recommend using the ASFR-weighted proportion to predict an NTD rate decline and measure the success of FA education campaigns. We found that when women in high-fertility age groups increased their FA consumption, the decline in the NTD rate was greater than when women in low-fertility age groups did so. Our findings suggest that the more efficient approach to NTD prevention is to focus on women with a higher probability of giving birth. For example, by focusing on <50% of women of childbearing age (20-34 years), as much as 76% of the maximum NTD rate reduction can be achieved.
authors.affiliation: Division of Birth Defects and Developmental Disabilities, National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, USA. qay0@cdc.gov
authors.forename: Quanhe
authors.initials: Q
authors.lastname: Yang
authors.forename: Michael
authors.initials: M
authors.lastname: Atkinson
authors.forename: J David
authors.initials: JD
authors.lastname: Erickson
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000465:Algorithms; D005260:Female; D005492:Folic Acid / Q000008:administration & dosage* / Q000627:therapeutic use; D005544:Forecasting; D006801:Humans; D008423:Maternal Age; D008875:Middle Aged; D015233:Models, Statistical; D009436:Neural Tube Defects / Q000178:diet therapy / Q000453:epidemiology / Q000517:prevention & control*; D011247:Pregnancy; D018566:Pregnancy, High-Risk
chemical_list: D005492:Folic Acid