Child-Mother Index: a new risk factor for selected adverse maternal birth outcomes

BACKGROUND: Over decades, obstetricians have evaluated a range of risk factors to improve the prediction of adverse birth outcomes. OBJECTIVE: This study aimed to assess the effectiveness of the Child-Mother Index as a risk factor indicator for selected adverse maternal birth outcomes. STUDY DESIGN: We assessed the Child-Mother Index by multinomial regression models using register-based data containing all singleton births in Denmark in 2009 with a gestational age between 37(+0) and 41(+6) weeks. The Child-Mother Index is defined as the weight of the newborn divided by the squared maternal height. RESULTS: Data from 47,007 births were included. Both the Child-Mother Index mean and Child-Mother Index median were 12.6 hg/m(2) (range, 4.8–22.4). In the multivariable model, the relative risk ratios for Child-Mother Index above 14.1 hg/m(2) were 2.2 (95% confidence interval, 1.6–3.1) for third- and fourth-degree perineal tears, 2.0 (1.6–2.5) for nonelective cesarean delivery, and 1.0 (0.8–1.3) for instrumental procedures. Equivalent figures for a Child-Mother Index below 11.2 hg/m(2) were 0.6 (0.4–1.0), 1.0 (0.8–1.2), and 0.7 (0.6–0.9), respectively. By comparing a multivariable model with the Child-Mother Index included with a model without the Child-Mother Index included using a likelihood ratio test, a statistically significant difference was found in favor of the Child-Mother Index inclusion (P<.001). CONCLUSION: The Child-Mother Index constitutes a potential useful risk factor indicator for statistical analyses on data after birth. The value of the Child-Mother Index based on the estimated fetal weight before birth deserves evaluation.