BMC Public Health 2005, 5:79 http://www.biomedcentral.com/1471-2458/5/79 affected the role of maternal age on infant mortality risk. Covariates that altered the odds ratio (OR) for the primary relationship by more than 10% were considered signifi- cant and included in the full model. 95% confidence intervals (CI) were computed to estimate the precision of the OR. First-order interaction terms between each covari- ate and maternal age were tested in a secondary analysis. The significance of adding each interaction was tested with a p-value <0.01. The final model was examined for outliers, influential data and goodness of fit (Omnibus Test and Hosmer and Lemeshow test, p <0.05). The role of each covariate in the full model was also assessed using neonatal and post-neonatal mortality as the primary out- come in separate logistic regression analyses. SPSS (Chi- cago, IL, version 10.0) was used for all statistical analyses. This research, reviewed by the Saint Louis University insti- tutional review board, was classified as exempt from the U.S. Department of Health and Human Services regula- tions for the protection of human subjects. The exemption 45 CFR 46.101(b)(4) permits epidemiologic research that uses existing publicly available data that are maintained in such a manner that subjects cannot be identified directly or through identifiers linked to the subjects. Results The distribution of covariates and outcomes for the three maternal age cohorts is reported in Table 1. Comparing all adolescent mothers (12 to 19 years) to older mothers, newborns of older mothers were, on average, 167 grams heavier than newborns of younger mothers (p < 0.001). Although the younger mothers gained 1.36 kg (3 pounds) on average more during pregnancy than older mothers (p < 0.001), this is not likely to be clinically significant. Compared to older mothers, younger mothers were more likely to be nulliparous (72.6% vs. 29.1%), smoke (27.2% vs. 18.2%), of black race (26.7% vs. 13.8%), and on Med- icaid (78% vs. 34.8%), WIC (75.1% vs. 36%), or food stamps (25.5% vs. 16.4%). Not surprisingly, fewer younger mothers were married (20.2% vs. 72.3%) and more had inadequate prenatal care (16.8% vs. 7%). Younger mothers were less likely to have an age-appropri- ate educational level (72.6% vs. 86.8%), and more likely to have a preterm infant (9.8% vs. 7.6%). Table 1: Characteristics of Missouri women with singleton live births, 1997–1999, by maternal age. 12 to 17 (N = 10,131) 18 to 19 (N = 18,954) 20–35 (N = 28,899) N % N % Characteristic Infant mortality Neonatal mortality Post-neonatal mortality Clinical Estimate of Gestational Age Birth Weight Category Maternal Race Age appropriate education level Medicaid W.I.C. Food Stamp Program Parity Unmarried Tobacco use During Pregnancy Alcohol use during pregnancy Kotelchuck index p ≤ 0.001 for all comparisons <24 Weeks 24–28 weeks 29–32 weeks 33–36 weeks >36 weeks Normal (> 2500 gm) Low (≤ 2500 gm) Missing/other Non-Hispanic white Non-Hispanic black nulliparous Unknown Inadequate Adequate/Intermediate Adequate Plus 121 69 52 40 102 161 808 8,961 9,114 1,017 137 6,688 3,306 8,592 7,698 7,761 2,060 8,660 9,140 2,486 61 487 2,101 5,273 2,270 1.2 0.7 0.5 0.4 1.0 1.6 8.0 89.0 90.0 10.0 1.4 66.0 32.6 86.1 77.1 77.8 20.7 85.5 90.3 24.7 0.6 4.8 20.7 52.0 22.4 158 92 65 50 110 252 1,318 17,109 17,358 1,596 256 14,234 4,464 12,516 14,983 14,085 5,351 12,375 14,064 5,425 90 747 2,789 11,121 4,297 0.8 0.5 0.3 0.3 0.6 1.3 7.0 90.8 91.6 8.4 1.4 75.1 23.6 66.0 80.4 75.6 28.7 65.3 74.3 28.7 0.5 3.9 14.7 58.7 22.7 N 173 111 60 59 139 260 1,739 26,571 27,207 1,692 635 24,288 3,976 25,086 10,070 10,412 4,729 8,382 7,986 5,262 268 805 2,037 18,832 7,225 % 0.6 0.4 0.2 0.2 0.5 0.9 6.0 92.4 94.1 5.9 2.2 84.0 13.8 86.8 35.4 36.7 16.7 29.0 27.7 18.3 0.9 2.8 7.0 65.2 25.0 Page 3 of 7 (page number not for citation purposes)