Influence of Birthweight on the Prospective Stillbirth Risk in the Third Trimester: A Cross-Sectional Cohort Study

OBJECTIVE The objective of this study was to determine the effect of birthweight on prospective stillbirth risk. METHODS Cross-sectional study of singleton births in the United States from 2010 to 2012 from 32 through 42 weeks was conducted. Stillbirth risk was stratified by birthweight and gestational age adjusted for time from death to delivery. The primary outcome was the prospective stillbirth risk for each birthweight category. Student t-test was used for continuous data, chi-square to compare categorical data. Binomial proportions were used to derive prospective and cumulative risks. Cox proportional hazards regression with log-rank test comparison for heterogeneity was used to compare birthweight categories and derive hazard ratios. RESULTS There was an increase in the risk for stillbirth as birthweight diverged from the reference group. At 40 weeks adjusted gestational age, stillbirth rate per 10,000 births for the bottom (6.17, 95% CI: 7.47-4.87) and top (2.37, 95%CI: 3.1-1.65) 5th centiles of birthweight conveyed the highest risk. Hazard ratios (HR) after adjusting for covariates were: 1.55 (1.73-1.4) <5th centile and 2.2 (2.43-1.99) > 95th centile (p < 0.001). CONCLUSION Stillbirth risk increases as birthweight departs from the mean. Birthweight below the 5th and above the 95th centile conveyed a significantly increased risk for stillbirth which was most noticeable after 37 weeks.