Adjusting National Health and Nutrition Examination Survey sample weights for women of childbearing age.

BACKGROUND Maternal risk factors have been tabulated for women of childbearing age using defined age ranges. However, statistics for factors strongly related to age may be overly influenced by values for the youngest and oldest women in a range, because pregnancies are most likely for ages 20-35. OBJECTIVE This report evaluates adjustment methods, based on the probability of pregnancy, for calculating estimates of risk factors for women of childbearing age. METHODS Adjusted and unadjusted estimates for environmental and nutritional variables were calculated from the 1999-2008 National Health and Nutrition Examination Survey (NHANES) for women aged 16-49. U.S. births were used to determine the probability of pregnancy. RESULTS Adjusted and unadjusted estimates differed for some, but not all, examined variables. More marked differences were observed for the environmental variables compared with the nutritional variables. Adjusted estimates were within about 5% of the unadjusted estimates for the nutritional variables. Adjusted geometric means for lead and mercury were about 7%-10% lower, and for polychlorinated biphenyl (or PCB) about 25% lower, than their respective unadjusted geometric means. With few exceptions, different adjustment methods led to similar estimates. CONCLUSION When calculating statistics for women of childbearing age, the decision to adjust for age or not to adjust appears to be more important than the choice of adjustment method. Although the results suggest only small differences among adjustment methods, approaches based on the NHANES design and sample weighting methodology may be the most robust for other applications.