| Objectives:To examine the independent impact of cesarean delivery in first pregnancy on neonatal mortality and morbidity in second pregnancy. This study was designed to provide evidence for clinical work and give some advise for decision-making on health strategy.Methods:A retrospective cohort study was conducted, based on the 1995-2002 linked live birth/infant death registration data of the United States compiled by the National Center for Health Statistics, Centers for Disease Control and Prevention. The linked registration data were based on all live births and infant deaths up to 1 year, registered in the 50 states and the District of Columbia. All of the study subjects were classified into two groups according to mode of delivery in first pregnancy:caesarean delivery in first pregnaney and vaginal delivery in first pregnancy. The prevalence rate of assisted ventilation,5 minutes Apgar score lower than 7, fetal distress, seizure, preterm, large for gestational age(LGA), small for gestational age(SGA) and neonatal death were compared between those two groups. Stratified analysis was performed to control confounding by indication of second second birth and exam if maternal health problems have modifying effects on the association between previous delivery method and neonatal outcomes. Maternal age, race, prenatal care began time, education year, smoking and child sex were adopted as covariates and multiple Poisson regression were used to estimate the association of model of delivery in the first pregnancy with neonatal morbidity and mortality in the second pregnancy. A subgroup of mothers of ethnic Chinese were selected to test if association in this subgroup population was consistence with overall population.Results:1) General characteristics:There were 31,713,906 births in the 1995-2002 linked birth and infant death data. Among them, 9,643,175 were second, singleton, and live births born between 24-44 weeks of gestation, with birth weight between 500-6500 gram. During 1995 to 2002, the average cesarean rate was 21.14% and showed upward trend by years. In 1995, the assisted ventilation and preterm rate were 23.1‰and 8.6%, and climbed to 25.4‰and 9.4% separately in 2002. Meanwhile,5 minute Apgar score lower than 7, seizure, fetal distress, LGA, SGA rate and neonatal mortality rate were declined from 8.9‰, 0.8‰,32.6‰,11.6%,8.9%,2.1‰to 7.7‰,0.4‰,27.1‰,10.9%,8.7%, 1.8‰separately, which showed statistical meaningful downward trend by years. According to the mode of delivery in first pregnancy, all of the study subjects were classified into two groups:previous cesarean delivery group and previous vaginal delivery group. Compared with previous vaginal delivery group, mothers with previous cesarean delivery tended to be older, married, non-Hispanic white, starting their prenatal care in first trimester, with higher education level, less tobacco usage during pregnancy and more likely being with medical complication in second pregnancy or adverse birth history in first pregnancy.2) Association between previous cesarean section and neonatal outcomes in overall population:The crude Risk Ratio for previous cesarean section and subsequent neonatal assisted ventilation,5 min Apgar score< 7, seizure, fetal distress, preterm, LGA, SGA, All-cause neonatal death, Asphyxia-related neonatal death were 1.47(95%CI:1.46, 1.49),1.14(95%CI:1.12,1.17),1.36(95%CI:1.27,1.45),1.46(95%CI: 1.44,1.47),1.00(95%CI:0.99,1.00),1.32(95%CI:1.32,1.33), 0.95(95%CI:0.95,0.96),1.01(95%CI:0.98,1.04)and 1.56(95%CI:1.38, 1.72) separately.According to the maternal medical complication and adverse birth history, we stratified all of the study subjects into 'healthy' and 'unhealthy' layer. We found that the association between subsequent neonatal 5 min Apgar score< 7, seizure, SGA, All-cause neonatal death, Asphyxia-related neonatal death and previous cesarean section were not modified by maternal medical complication or adverse birth history in first pregnancy. However, the association between assisted ventilation, fetal distress, preterm, LGA and previous cesarean section were all modified by maternal medical complication and adverse birth history.After the adjustment for maternal age, race, prenatal care began time, year of education, smoking, infant sex, preterm, maternal medical complication and adverse birth history in first pregnancy, the association between subsequent neonatal 5 min Apgar score< 7, seizure, Asphyxia-related neonatal death and previous cesarean section still exist and the adjusted Risk Ratio were 1.13 (95%CI:1.11,1.15),1.36 (95%CI: 1.27,1.46) and 1.52 (95%CI:1.36,1.71) separately. The relationship between All-cause neonatal death and previous C-section is statically insignificant (adjusted RR=1.03,95%CI:0.99,1.07). The adjusted Risk Ratio for SGA and previous C-section is 0.98(95%CI:0.98,0.99).After considering the modification effects of maternal medical or adverse birth history, we estimated the association between previous cesarean delivery and assisted ventilation, fetal distress, preterm, LGA in two layers separately. Compared with previous vaginal delivery group, infants born to mothers with a previous cesarean delivery had higher rates of assisted ventilation, fetal distress and LGA in both layers:in 'unhealthy'layer, the adjusted Risk Ratio for previous cesarean and ventilation, fetal distress were 1.43(95%CI:1.42,1.45),1.43(95%CI: 1.42,1.44) and 1.29(95%CI:1.28,1.30) separately; in 'healthy' layer, the adjusted Risk Ratio were 1.39(95%CI:1.35,1.43),1.11(95%CI: 1.09,1.14) and 1.58(95%CI:1.56,1.61) separately.The association between preterm and previous cesarean section showed different strength. Previous cesarean delivery is not associated with preterm in'healthy'layer (adjusted RR=1.00,95%CI:0.99,1.00), but has statistical association with preterm in'unhealthy'layer (adjusted RR=1.03,95%CI:1.02,1.05).3) Association between previous cesarean section and neonatal outcomes in Chinese subgroup population:After the adjustment of maternal age, race, prenatal care began time, year of education, smoking, infant sex, preterm, maternal medical complication and adverse birth history in first pregnancy, the relationship between assisted ventilation, fetal distress, preterm, LGA, SGA and previous cesarean delivery in Chinese race subgroup are consistence with overall population, the Risk Ratio were 1.52(95%CI:1.30,1.77),1.75(95%CI:1.56,1.97), 1.06(95%CI:0.99,1.14),1.43(95%CI:1.34,1.53) and 0.86(95%CI:0.80, 0.93) separately. The association between seizure, asphyxia-related neonatal death and previous cesarean delivery are statistically insignificant in Chinese race subgroup, which is different from overall population. Compared with previous vaginal delivery group, infants bom to mothers with a previous cesarean delivery had lower risk of having SGA infant in second pregnancy, which is consistent with the association in'healthy'layer.Conclusions:Compared with previous vaginal delivery group, infants born to mothers with a previous cesarean delivery had higher rates of assisted ventilation,5 minte Apgar score lower than 7, seizure, fetal distress and LGA. Those relationships are not all contributed to maternal medical complication or adverse birth history. Previous cesarean delivery is associated with preterm in second pregnancy only among those with maternal medical complication or adverse birth history population. In Chinese race subgroup population, the relationship between assisted ventilation, fetal distress, preterm, LGA and previous cesarean delivery still exist. Clinicians should be cautious on a decision for cesarean delivery, especially those when there is no clear medical indication for cesarean section.
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