Objective The authors wanted to assess the risk of stillbirth in different categories, and study their association with the rate of small-for-gestational age (SGA) babies.
Method 48 357 consecutive births during 2006/7 from six maternity units in the West Midlands. Rates of SGA at birth were assessed by two methods: SGA based on the local weight for gestational age standard (‘population SGA’), or the individually adjusted fetal growth potential (‘customised SGA’).
Results 10.5% of mothers had a body mass index (BMI) <20, and 7.8% a BMI of 35+. Stillbirth rates (per 1000) rose incrementally from low to high BMI categories: BMI<20: 2.8; 20–24.9: 6.2; 25–29.9: 7.6; 30–34.9: 8.5; 35+: 9.8. SGA rates (%) based on the customised fetal growth potential in the five BMI groups were 18.5, 15.9, 17.1, 18.8 and 22.0, respectively, and appeared to follow the increasing stillbirth rate. However in the BMI<20 group, the SGA rate was higher despite a lower risk of stillbirth. In contrast, the rate of SGA by population based centiles demonstrated a downward trend as BMI increased, with the lowest SGA rate being in the highest BMI category.
Conclusion The elevated stillbirth risk in pregnancies of obese mothers is associated with an increase in customised SGA rate. This link is hidden when population based centiles are used, but becomes apparent when birthweight is measured against the customised growth potential, thus helping to identify the presence of fetal growth restriction.
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