Birth weight differences between preterm stillbirths and live births: analysis of population-based studies from the U.S. and Sweden

BACKGROUND Many stillbirths show evidence of fetal growth restriction, and most occur at preterm gestational age. The objective of this study is to compare birth weights at preterm gestational ages between live births and stillbirths, and between those occurring before or during labour. METHODS Based on singleton births from the United States (U.S.) 2003-2005 (n=902,491) and Sweden 1992-2001 (n=946,343), we compared birth weights between singleton live births and stillbirths at 24-36 completed weeks of gestation from the U.S. and at 28-42 completed weeks from Sweden. RESULTS In both the U.S. and Sweden, stillbirth weight-for-gestational-age z-scores were at least one standard deviation lower than live birth z-scores at all preterm gestational ages (GA). In Sweden, no birth weight difference was observed between antepartum and intrapartum stillbirths at preterm GAs, whereas birth weights among intrapartum stillbirths were similar to those among live births at 37-42 weeks. CONCLUSIONS Birth weights observed at preterm gestation are abnormal, but preterm stillbirths appear to be more growth-restricted than preterm live birth. Similar birth weights among ante- and intrapartum preterm stillbirths suggest serious fetal compromise before the onset of labor.