A population study of the relationship between fetal death and altered fetal growth.

In order to describe the relationship between fetal death rate and impaired fetal growth, we examined over 850,000 births in Illinois between 1980-1984 (using the state computer data file) and assessed the mean/modal birth weights at each gestational age and the relationship between birth weight and fetal death rate at each gestational age. We were interested in the following questions: 1) Is the relationship between impaired fetal growth and fetal death rate the same at each gestational age? and 2) What birth weight would result in a quadrupling of the fetal death rate at each gestational age? Using exponential regression analysis, we determined for each gestational age the fetal death rate at the modal birth weight and similarly, the birth weight expected to result in a quadrupling of the fetal death rate. As gestational age advanced, the birth weight percentile resulting in the constant outcome also increased (second percentile at 25 weeks; 17th percentile at 42 weeks). We also compared these data with similar data from Denver. The findings indicate the following: 1) Fetal death rate increases exponentially as birth weight decreases at each gestational age; 2) the birth weight percentile that results in a constant outcome is not consistent at each gestational age; and 3) if assessment of risk is to be inferred based on the relationship between birth weight and gestational age, the tenth percentile (whether Denver, Illinois, or elsewhere) does not predict stillbirth accurately. The implications point to the use of outcome-oriented risk assessments to predict fetal death when examining the relationship between birth weight and gestational age.