was obtained from HCUP and from the Institutional Review Board (New York, USA). We reviewed the NIS database from January 2008 to December 2008 and identified 1,204,887 live births (i.e., birth hospitalizations). We included 1,014,261 (84%) live births with available sex and gestational age data in our final cohort. All cases of CM diagnoses during birth hospitalization were identified by ICD9 code 740.0e759. These diagnoses were made clinically or by autopsy of infants of live births that died during birth hospitalization. To avoid double counting, we restricted our inclusion criteria to CMs diagnosed during birth hospitalization. We ensured this by including only hospitalizations with ICD-9 code for normal and complicated delivery (650.0e669.0); hence, this excluded diagnoses made during interhospital transfer or during readmission hospitalization. In patients with multiple CMs, each malformation was counted separately. We grouped all CMs by different organ systems. Based on the classification system by Christensen et al,13 we defined multiple organ system involvement as live births with CMs that involved two or more organ systems. For gestational maturity, the NIS coding system defined preterm birth and term birth as delivery before and after 37 completed weeks of gestation, respectively. We analyzed 62 selected CM diagnoses to determine the effect of sex and gestational maturity on the birth prevalence of CM. For the odds ratio calculation, we considered males as the exposed group for sex, and preterm births as the exposed group for gestational maturity. We excluded genitourinary malformations from our sex analysis because of differences between the sexes in the spectrum of genital malformations. Data weighting was performed with SAS software (NC, USA) in accordance with the HCUP recommendations.14 The NIS has undergone some changes over time in sampling strategy, weighting strategy, and data element available. We adjusted for these changes in accordance with the recommendations in the NIS Trend Supplemental files available at http://www.hcup-us.ahrq.gov/db/nation/nis/ nistrends.jsp. 15 This analysis excluded CM diagnoses with a cell size of 10 or fewer in keeping with the HCUP data use agreement, which prohibits reporting cell sizes of 10 or fewer. The CM prevalence was expressed per 1000 live births and then stratified by sex and gestational maturity. We used the Chi-square test to assess differences between groups. A p value of 1.2 million. We believe that our estimates are more representative of the true CM prevalence in the general newborn population because of Table 1 Birth prevalence of congenital malformations. No. of cases (incidence per 1000 births) Total Male Female OR (CI) Term Preterm OR (CI) Patients with confidence interval; CM Z congenital malformation diagnosis (ICD9 codes 740.0e759.9); Isolated NSCM Z isolated nonsyndromic congenital malformations [all CM diagnoses, excluding the genetic syndromes (ICD9 codes 740.0e757.9 and 759.0e759.9)]; Multiple NSCM Z nonsyndromic congenital malformations involving two or more organ systems; OR Z odds ratio; Syndromic CM Z all genetic syndromes (ICD9 codes 758.0e758.9). * Indicates statistical significance. Table 2 Relative distribution of congenital malformations (excluding genetic syndromes). Congenital malformation No. of cases % Neurologic 1488 4.7 Craniofacial 1108 3.5 Cardiac 11,243 35.5 Respiratory 2755 8.7 Gastrointestinal 3198 10.1 Genitourinary 8778 27.7 Musculoskeletal 2185 6.9 Others 918 2.9 Total CM (excluding GS) 31,673 Total CM 32,845 Craniofacial Z eye and facial malformations; Neurologic Z brain and spinal malformations; Others Z spleen and abdominal wall anomalies; Total CM Z total congenital malformation; Total CM (excluding GS) Z total congenital malformations, excluding genetic syndromes (this is the denominator). Congenital Malformations in the Newborn Population 27 Table 3 Birth prevalence of congenital malformations. derived from a national database rather a regional database. Our study also showed a significantly higher risk of overall CM in infants of preterm births. An increased risk was also observed for multiple organ system involvement and these findings are consistent with previous studies.16e18 We speculate that this finding could be the result of two factors. The first factor is a difference in the ascertainment rate. Preterm newborns spend more time in the intensive care unit and are subjected to more diagnostic tests, compared to term newborns. This increases the odds of diagnosing subtle CMs in preterm newborns than in term newborns. Another possible explanation for our finding may be that CM and prematurity may share some underlying maternal risk factors such as smoking, obesity, hypertension, and diabetes mellitus.19e22 Hence, the presence of a CM could be part of a causal mechanism that leads to preterm delivery. Epidemiology of CMs is well studied and has been previously reported in multiple studies.1,9,10,17,18 However, there are certain novel aspects to our study that makes it different from previous publications. First, we studied the prevalence of CMs among live births in the United States, excluding stillbirths and abortions. Most population-based studies of CM prevalence have included live births, stillbirths, and abortions in their