Fetal growth risk curves: Defining levels of fetal growth restriction by neonatal death risk

Objective We developed a fetal growth risk curve that delineates the birth weight values for gestational age that reflect a 2-, 2.5-, and 3-fold neonatal death risk relative to infants with normal fetal growth. Study design We analyzed 18,085,052 single gestation infants (25-42 weeks) who were born to US resident mothers from 1996 to 2000. Multivariate models were used to predict the relationship between neonatal death and birth weight percentile. Fetal risk curves were derived on the basis of birth weight percentile–specific neonatal mortality rates that were relative to an average rate of neonatal death for a comparison group that was representative of typical growth (ie, infants between 45th-55th birth weight percentiles for gestational age). Results The 10th percentile of birth weight for gestational age is associated with an increased but variable risk of neonatal death relative to the comparison group across the spectrum of gestational ages. At 26 weeks of gestation, infants at the 10th percentile experienced a 3-fold risk of dying within the first 28 days of life (relative to the comparison group); whereas at 40 weeks, the risk was 1.13. Conclusion Fetal growth risk curves facilitate the identification of populations of infants whose risk of death are deemed excessive compared with that of infants at the norm of fetal growth and may be useful for counseling pregnant women.