Objective To investigate the influencing factors of low birth weight infants (LBWI) in China in order to provide evidence for lowering the incidence of LBWI and improving the perinatal outcomes. Methods Clinical data were obtained from 14 different provinces, municipalities or autonomous regions in Northeastern, Northwestern, Northern, Central, Eastern, Southern and Southwestern of China, covering 39 hospitals of different levels. A total of 112 441 newborns were collected from January 1 to December 31 in 2011. After exclusion of those cases with incomplete information, miscarriage before 28 weeks of gestation, induction due to fetal malformation or intrauterine fetal death, 103 678 cases were restrospectively analyzed. Questionnaires were filled out and all data were recorded in computer network databases. Clinical data included maternal age, education background, height, weight, parity, histories of abnormal pregancy and comorbidities and complication. Independent sample t-test, Chi-square test, unvariate and ultivariate unconditional Logistics regression analysis were performed. Results The incidence of LBWI in mainland China was 7.21% (7 474/103 678), 61.43% (5 260/8 562) for preterm babies, and 2.33% (2 214/95 116) for full-term babies. Univariate analysis showed that LBWI were associated with maternal age, education background, height, pregestational body mass index (BMI), weight gain during pregnancy, cord length, smoking, parity, histories of abnormal pregancy, gestational diabetes mellitus (GDM), preterm birth, hypertensive disorders in pregnancy, anemia, premature rupture of membranes and abnormal amniotic fluid volume. The following unconditional binary logistic regression analysis for those factors with P<0.3 in unvariate analysis showed that preterm birth (OR=46.246, 95%CI: 41.484-51.555), hypertensive disorders during pregnancy (OR=5.031, 95%CI: 4.325-5.853), histories of intrauterine fetal death ≥1 times (OR=2.446, 95%CI: 1.479-4.044), oligohydramnios (OR=2.068, 95%CI:1.659-2.578), pregestational BMI<18.5 (OR=1.637, 95%CI: 1.415-1.893), spontaneous abortion ≥1 times (OR=1.362, 95%CI: 1.043-1.777), age≤20 (OR=1.332, 95%CI: 1.046-1.695), anemia (OR=1.230, 95%CI: 1.017-1.488) and premature rupture of membranes (OR=1.154, 95%CI: 1.016-1.311) were risk factors for LBWI. The higher the maternal education level, weight gain, BMI and height, the lower the LBWI incidence. The risk factors of LBWI in premature small for gestational age (SGA) infants were hypertensive disorders during pregnancy and histories of intrauterine fetal death ≥1 times. The higher the maternal height and weight gain during pregnancy, the lower the incidence of LBWI in premature SGA infants. Conclusions The main influencing factors for LBWI are preterm birth and hypertensive disorders during pregnancy. In addition, LBWI is also associated with socioeconomic and genetic factors. Key words: Infant, low birth weight; Incidence; Risk factors
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