**5. Influence of prepregnant BMI and GWG on birth weight**

### **5.1 SGA infant**

Low prepregnancy BMI and low GWG have been associated with the delivery of smaller infants. There is strong evidence for an association between weight gains below the IOM guidelines and the risk of having an SGA infant ( Nielsen et al., 2006; Park et al., 2011). In a study undertaken of 26,028 women in California, those with gestational weight gains below the IOM guidelines had a significantly higher risk of SGA infants when compared with women with a weight gain above IOM guidelines, adjusting for prepregnancy BMI (Park et al., 2011).

Similar results were also observed in a study among Swedish women with singleton fullterm births. The risks for SGA were higher when gaining <8 kg (95% CI: 1.68-2.35) and lower when gaining >16 kg (95% CI: 0.50-0.61) (Cedergren, 2006). Merchant et al. (1999) reported lower mean birth weights of newborns among Pakistani women with prepregnancy BMI<19 kg/m2 who gained >12.5 kg compared to those who gained 12.5 kg. Women in the lowest quartile for both prepregnancy BMI and GWG were 5.6 times more likely to have intrauterine growth restriction (IUGR) infants, compared to women in the upper quartile (Naidu & Rao, 1994). The research among 3,071 Japanese women who gave birth to single-term infants found that underweight women were 1.7 times and 1.5 times more likely to give birth to a SGA or LBW infant, respectively (Watanabe et al., 2007). However, an increase in GWG eliminated or reduced the incidence of SGA.

Current evidence indicates that GWG, particularly during the second and third trimesters, is an important determinant of fetal growth (Althuizen et al., 2006; Kaiser et al., 2008). Inadequate weight gain during these trimesters is associated with an increased risk of LBW or IUGR. Health care providers should give women individual graphs of their weight gains at each antenatal check up, having viewed valuable sources of information on diet and nutrition.

#### **5.2 LGA infant**

The mean birth weight has continuously increased in United States, Canada, Europe, and Asia (Kramer et al., 2002). In Australia, LGA births have increased from 9.2% to 10.8% in male infants and from 9.1% to 11% in female infants from 1990 to 2005 (Hadfield et al., 2009). The 25% to 36% increase in maternal BMI over the past decade has translated to approximately a 25% increase in the incidence of newborns with high birth weight (Surkan et al., 2004). The possible reason may be increased maternal body weight and/or excess weight gain during pregnancy beyond the recommended IOM guideline. Strong evidence confirms the association between excessive GWG and increased birth weight in all BMI categories.

A population-based retrospective cohort study was reported of 570,672 women aged 18-40 years, examining the association between the 2009 IOM recommendations and adverse infant outcomes by prepregnancy BMI reported that fifty-one percent of women were above the IOM guidelines. Gains of greater than the recommended amount were associated with increased odds of LGA (95% CI: 1.27-5.99), and gains less than those recommended were associated with decreased odds of LGA (95% CI: 0.27-0.77) (Park et al., 2011). Similar findings observed that the proportions of LGA increased with high prepregnancy BMI, but the proportions of SGA decreased. Obese women with lower weight gain or weight loss during pregnancy had lower risk of LGA and higher risk of SGA (Nohr et al., 2008).
