Background: The purpose of the paper was to determine the risk factors for small-for-gestational-age (SGA) infants at full term, in Japan. Methods: The study was conducted at four hospitals and clinics in the Tokyo metropolitan area. A retrospective review of 2972 mothers and their infants born from singleton pregnancies at any time during the years 2002 and 2003 was conducted. Results: Of these women, 8.4% gave birth to SGA infants. The proportion of SGA infants was significantly higher among heavy smokers (>10 cigarettes/day; 13.7%, P < 0.01). The odds ratio (OR) for SGA decreased significantly in proportion to the pregnancy body mass index (OR, 0.89; 95% confidence interval [CI]: 0.84-0.94, P < 0.001). The OR of SGA for stratified maternal weight gain was 1.79 (95%CI: 1.24-2.58, P ≤ 0.01) for weight gain < 8.0 kg; 1.16 (95%CI: 0.79-1.71, P = 0.45) for weight gain 8.0-10.0 kg; and 0.49 (95%CI: 0.3-0.78, P < 0.01) for weight gain >12 kg. Conclusion: The present study clearly confirms the detrimental effect of a low prepregnancy body mass index, low maternal weight gain and maternal smoking during pregnancy on the incidence of SGA infants.
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