FM047 Population based vs customised fetal weight standards for the identification of maternal BMI related stillbirth risk
Fetal Medicine
Information
Objective High maternal body mass index (BMI) is associated with increased stillbirth risk, yet is also claimed to be protective of SGA1. We wanted to examine this association using population based and customised charts for fetal growth used in the UK.
Method The cohort consisted of 2,554,665 British-European pregnancies delivered in NHS hospitals from 2015 to 2025. Stillbirth rates (24+ weeks) and SGA rates (<10th centile) at birth were calculated in 5 maternal BMI groups. Population charts were the Fetal Medicine Foundation (FMF) 2 and the revised Intergrowth-21st (IG21)3 standards. The Gestation Related Optimal Growth (GROW) standard 4 was customised for maternal height, weight, parity and ethnic origin. Trend of SGA rates were compared with stillbirth rates across the BMI groups using Clogg’s Z test.
Results The average stillbirth rate was 3.80 and had a U shaped distribution across the 5 BMI categories (Figure 1). SGA rates varied widely for the two population based charts (at BMI 18.5-25.0: FMF=19.0, IG21=8.6%). Both population standards had a high SGA rate at low BMI, and a downward trend with increasing BMI, contrary to the stillbirth rate which increased with obesity (p<0.05). In contrast, SGA rates according to GROW followed the stillbirth rate across BMI groups.
Conclusion Customised charts identify an association between increasing maternal BMI and risk of SGA associated stillbirth risk. Consistent with previous findings 5, these population charts in current use hide the increased SGA rate in high BMI mothers, and therefore may miss growth restriction associated stillbirth risk. 1. Cnattingius S, Bergström R, Lipworth L, Kramer MS. Prepregnancy weight and the risk of adverse pregnancy outcomes. N Engl J Med 1998;338:147-52. https://doi.org/10.1056/NEJM199801153380302
2. Nicolaides KH, Wright D, Syngelaki A, Wright A, Akolekar R. Fetal Medicine Foundation fetal and neonatal population weight charts. Ultrasound Obstet Gynecol. 2018;52(1):44-51.
https://doi.org/10.1002/uog.19073
3. Stirnemann J, Salomon LJ, Papageorghiou AT. INTERGROWTH-21st standards for Hadlock’s estimation of fetal weight. Ultrasound Obstet Gynecol. 2020;56(6):946–8. https://doi.org/10.1002/uog.22000
4. Gestation Related Optimal Weight (GROW) customised growth chart; Perinatal Institute 2018-2025. https://www.perinatal.org.uk/FetalGrowth
5. Gardosi J, Hugh O. Stillbirth risk and smallness for gestational age according to Hadlock, Intergrowth-21st, WHO and GROW fetal weight standards: analysis by maternal ethnicity and body mass index. Am J Obstet Gynecol. 2023;229(5):p547.e1-547.e13.
https://doi.org/10.1016/j.ajog.2023.05.026
Method The cohort consisted of 2,554,665 British-European pregnancies delivered in NHS hospitals from 2015 to 2025. Stillbirth rates (24+ weeks) and SGA rates (<10th centile) at birth were calculated in 5 maternal BMI groups. Population charts were the Fetal Medicine Foundation (FMF) 2 and the revised Intergrowth-21st (IG21)3 standards. The Gestation Related Optimal Growth (GROW) standard 4 was customised for maternal height, weight, parity and ethnic origin. Trend of SGA rates were compared with stillbirth rates across the BMI groups using Clogg’s Z test.
Results The average stillbirth rate was 3.80 and had a U shaped distribution across the 5 BMI categories (Figure 1). SGA rates varied widely for the two population based charts (at BMI 18.5-25.0: FMF=19.0, IG21=8.6%). Both population standards had a high SGA rate at low BMI, and a downward trend with increasing BMI, contrary to the stillbirth rate which increased with obesity (p<0.05). In contrast, SGA rates according to GROW followed the stillbirth rate across BMI groups.
Conclusion Customised charts identify an association between increasing maternal BMI and risk of SGA associated stillbirth risk. Consistent with previous findings 5, these population charts in current use hide the increased SGA rate in high BMI mothers, and therefore may miss growth restriction associated stillbirth risk. 1. Cnattingius S, Bergström R, Lipworth L, Kramer MS. Prepregnancy weight and the risk of adverse pregnancy outcomes. N Engl J Med 1998;338:147-52. https://doi.org/10.1056/NEJM199801153380302
2. Nicolaides KH, Wright D, Syngelaki A, Wright A, Akolekar R. Fetal Medicine Foundation fetal and neonatal population weight charts. Ultrasound Obstet Gynecol. 2018;52(1):44-51.
https://doi.org/10.1002/uog.19073
3. Stirnemann J, Salomon LJ, Papageorghiou AT. INTERGROWTH-21st standards for Hadlock’s estimation of fetal weight. Ultrasound Obstet Gynecol. 2020;56(6):946–8. https://doi.org/10.1002/uog.22000
4. Gestation Related Optimal Weight (GROW) customised growth chart; Perinatal Institute 2018-2025. https://www.perinatal.org.uk/FetalGrowth
5. Gardosi J, Hugh O. Stillbirth risk and smallness for gestational age according to Hadlock, Intergrowth-21st, WHO and GROW fetal weight standards: analysis by maternal ethnicity and body mass index. Am J Obstet Gynecol. 2023;229(5):p547.e1-547.e13.
https://doi.org/10.1016/j.ajog.2023.05.026