Objective: The fetoplacental ratio has been used conventionally to study the contribution of the placenta to fetal growth restriction. However, this measure is problematic because a normal fetoplacental ratio can reflect birth weight and placental weight that are both normal, both low, or both high. The objective of this study was to examine the independent association between placental weight for gestational age and perinatal mortality or serious neonatal morbidity.
Methods: A sex- and gestational age-specific placental weight z score was calculated for a cohort of 87,600 singleton births at the Royal Victoria Hospital in Montreal, Canada, 1978-2007. The relationship between placental weight z score and adverse perinatal outcomes (stillbirth, neonatal death, 5-minute Apgar score lower than 7, seizures, or respiratory morbidity) was examined using logistic regression. Multivariable models examined whether the relationship was independent of birth weight and other pregnancy risk factors.
Results: : After controlling for birth weight, fetuses with a low placental weight z score were at significantly increased risk of stillbirth (odds ratio [OR] 2.0, 95% confidence interval [CI] 1.4-2.6, percent population attributable risk 17.8%). In contrast, adverse neonatal outcomes were significantly more likely among those with high placental weight z scores (OR 1.4, 95% CI 1.2-1.7, percent population attributable risk 5% for any serious neonatal morbidity). Similar trends were observed after further adjusting for pregnancy risk factors.
Conclusion: Placental weight for gestational age is an independent risk factor for adverse perinatal outcomes, above and beyond the known association with birth weight. The mechanisms behind the opposing effects of placental weight z score on risk of stillbirth compared with adverse neonatal outcomes require further elucidation.
Level of evidence: III.