BMC Public Health 2004, 4 http://www.biomedcentral.com/1471-2458/4/16 Table 3: Number of households with at least one person with a birth defect and rates (per 10,000) identified in the NHIS (1994/1995) by area of residence and projection to the total U.S. population in 2000. *Source: Fields JM, Casper LM. America's Families and Living Arrangements: March 2000. Current Population Reports, P20-537, U.S. Census Bureau, 2001. Total Number of Households by Region Northeast Midwest South West Households with single non-familial birth defect Northeast Midwest South West Households with multiple non-familial birth defects Northeast Midwest South West Households with single familial birth defect Northeast Midwest South West Households with multiple familial birth defects Northeast Midwest South West n (rate) 28,094 5,604 6,825 9,568 6,097 1,083 (385.5) 212 (378.3) 218 (319.4) 388 (405.5) 265 (434.6) 55 (19.6) 9 (16.1) 18 (26.4) 15 (15.7) 13 (21.3) 54 (19.2) 10 (17.8) 15 (22.0) 14 (14.6) 15 (24.6) 8 (2.8) 3 (5.4) 3 (4.4) 2 (2.1) 0 Projections Prevalence rates were calculated based on the number of households identified in each group and the total number of households asked about these specific conditions (28,094). We did not use a weighting strategy since the purpose of our study was to describe population trends rather than to calculate precise estimates of prevalence as done in other studies using the NHIS[9]. Instead, to esti- mate the total number of households where there are two or more family members with a birth defect, the preva- lence rates calculated from the NHIS were applied to the data of the U.S. Census Bureau report "America's Families and Living Arrangements, March 2000"[10] (Table 3). This document includes trends about households, fami- lies, living arrangements, characteristics of single-parent families, differences in the living arrangements of younger and older adults, and data on unmarried-couple house- holds. A comparison of demographic characteristics between the NHIS and the Census report data confirmed that the NHIS sample is an excellent representation of all U.S. households (Table 4). Total Projected U.S. 2000* 104,706,000 20,087,000 24,508,000 37,303,000 22,808,000 4,036,328 759,894 782,820 1,512,705 991,327 204,984 32,260 64,636 58,481 48,631 201,257 35,844 53,864 54,582 56,113 29,816 10,753 10,773 7,797 - Frequencies and unadjusted odds ratios and their 95% confidence intervals were calculated with SAS version 8[11] and Stata version 8[12]. Results Table 1 shows the number and type of conditions found in the four groups of households. Of a total of 28,094 households surveyed for birth defects and impairments, there were 1,083 single non-familial, 55 multiple non- familial (total of 115 affected individuals), 54 single familial (total of 113 affected individuals), and 8 multiple familial (total of 16 affected people) households. Close to 60 percent of all birth defects in these groups were of the musculoskeletal system, such as musculoskeletal deform- ities of the skull, face, and jaw (including the sternocleid- omastoid muscle); chondrodystrophy; osteodystrophies; congenital scoliosis; clubfoot; and flat foot/congenital fallen arches. The other most common conditions were congenital heart defects (CHD), spina bifida and hydro- cephalus, and oral clefts. CHDs were particularly frequent in the multiple familial and in the single non-familial groups (26.9% and 20.8% of all the congenital anomalies in each group, respectively). Page 4 of 7 (page number not for citation purposes)