RÉSUMÉ
<p><b>OBJECTIVE</b>To investigate the relationship between maternal pre-pregnancy body mass index, weight gain during pregnancy and small for gestational age (SGA) birth so as to provide evidence for the development of comprehensive prevention programs on SGA birth.</p><p><b>METHODS</b>Between March, 2012 and July, 2014, 4 754 pregnant women were asked to fill in the questionnaires which were collected from the First Affiliated Hospital of Shanxi Medical University. Data related to general demographic characteristics, pregnancy and health status of those pregnant women was collected and maternal pre-pregnancy body mass index and maternal weight gain were calculated. Subjects were divided into different groups before the effect of maternal pre-pregnancy body mass index and weight gain during pregnancy on SGA birth were estimated.</p><p><b>RESULTS</b>The overall incidence of SGA birth was 9.26% (440/4 754). Proportions of SGA birth from pre-pregnant, underweight group, normal weight group, overweight and obese groups were 9.85%, 8.54% and 9.45%, respectively. Results from multi-factor logistic regression analyses showed that after adjusting the confounding factors as age, history on pregnancies etc., women with high pre-pregnancy BMI showed a lower incidence of SGA than those under normal pre-pregnancy BMI (OR = 0.714, 95% CI: 0.535-0.953). Different weight gains during pregnancy were statistically significant (χ(2) = 8.811, P = 0.012). Incidence of SGA birth that was below the recommended range in the 2009 Institute of Medicine Guidelines (12.20%) was higher than those within (9.23%) or beyond (8.45%) the recommended range. Results from the multi-factor logistic regression analyses showed that, after adjusting the confounding factors as age, pregnancy history etc., factor as weight gain below the recommended level could increase the risk of SGA (OR = 1.999, 95% CI: 1.487-2.685). In the underweight, normal weight, overweight or obese groups, with weight gain during pregnancy below the range, the incidence of SGA showed an increase (OR = 2.558, 95% CI: 1.313-4.981, OR = 1.804, 95% CI: 1.258-2.587, OR = 3.108, 95% CI: 1.237-7.811). There was no interaction of addictive or multiplicative models between these two factors under 'interaction analysis'.</p><p><b>CONCLUSION</b>Women with high pre-pregnancy BMI presented a lower incidence of SGA than those within the normal range. Insufficient weight gain during pregnancy could increase the risk of SGA delivery. These findings called for attention to be paid to the gestational weight gain, in order to decrease the risk of SGA.</p>