Defining poor growth and stillbirth risk in pregnancy for infants of mothers with overweight and obesity.

Background: Mothers who are obese carry heavier fetuses and have lower rates of small for gestational age (<10th birthweight centile) infants. However, their infants may be growth-restricted (with an increased risk of stillbirth) at a higher birthweight centile compared with infants from healthy-weight women.

Objective: This study aimed to quantify the birthweight centile at which the risk of stillbirth in infants born to obese women equaled that of <10th-centile infants born to healthy-weight women, and clarify the relationship between maternal body mass index, infant size, and stillbirth.

Study Design: We conducted a retrospective cohort study on all infants born in Victoria, Australia, from 2009 to 2019 (754,946 cases for analysis). We applied uncustomized birthweight centiles to all infants, and stratified the maternal cohort by body mass index (<20 kg/m, 20-25 kg/m, 25-30 kg/m, 30-35 kg/m, 35-40 kg/m, ≥40 kg/m). For each body mass index category, we assessed proportions of infants born <10th centile and <3rd centile, stillbirth rates among infants of all sizes, and small for gestational age infants. We calculated the stillbirth rate (per 1000) and relative risk (risk of stillbirth if born <10th centile vs >10th centile) for healthy-weight women (body mass index, 20-25 kg/m). We then determined the birthweight centile for infants born to mothers within other body mass index categories that equaled that rate or risk.

Results: Stillbirth rates increased with increasing maternal body mass index. Infants classified as small for gestational age (<10th centile) from mothers with high body mass index had a higher risk of stillbirth (relative risk, 3.15; 95% confidence interval, 2.22-4.47; for mothers with body mass index ≥40 kg/m vs healthy-weight mothers [body mass index, 20-25 kg/m]). The stillbirth rate (stillborn infants per 1000 births) among <10th-centile infants born to healthy-weight mothers was 7.5 per 1000. The same stillbirth rate was observed at higher birthweight centiles for infants of women with higher body mass index (<18th centile for those with a body mass index of 25-30 kg/m, <25th centile for body mass index of 30-35 kg/m, <31st centile for body mass index of 35-40 kg/m, <41st centile for body mass index of ≥40 kg/m). The relative risk of stillbirth among small for gestational age infants of healthy-weight mothers was 5.46 (95% confidence interval, 4.65-6.40). The birthweight centile with a comparable relative risk of stillbirth increased with increasing body mass index (<16th centile for women with body mass index of 25-30 kg/m, <19th centile for body mass index of 30-35 kg/m, <28th centile for body mass index of 35-40 kg/m, <30th centile for body mass index ≥40 kg/m).

Conclusion: Obesity affects the relationship between infant size and perinatal mortality. The stillbirth risk observed in <10th-centile infants from healthy-weight mothers occurs at higher birthweight centiles with overweight or obese mothers. Clinicians should be aware that the same infant risk exists at a higher birthweight centile for women with higher body mass index.