Fetal growth and birthweight standards as screening tools: methods for evaluating performance.

OBJECTIVE: To discuss different methods for evaluating fetal growth and population-based birthweight standards relevant to different uses: either in antenatal care or in epidemiology.
DESIGN: Population-based cohort study.
SETTING: Routinely collected data in Scotland. POPULATION: A total of 540,849 singletons born after 24 weeks between 1980 and 2003.
METHODS: The performance of a fetal growth standard and a population-based birthweight standard are compared in two ways. First, we consider the accuracy of estimated risks of stillbirth at any point during the remaining pregnancy, a measure that is relevant in antenatal care. Second, the rates of stillbirth at each gestation, which are measures relevant in epidemiology, are compared with the actual rates. MAIN OUTCOME MEASURES: Standard measures of screening and diagnostic performance: sensitivity, specificity, and positive and negative predictive values.
RESULTS: In clinical care, the evidence points towards using fetal growth standards: sensitivity at term is about 30%, increasing to 43% for preterm births (24-31 weeks of gestation), compared with about 29% across all ages under the birthweight standard. Positive predictive values are about 1% across gestations. For epidemiology, the evidence is not so clear-cut: preterm, the population birthweight standard has higher sensitivity and specificity, but this is not the case in the full-term weeks.
CONCLUSIONS: The performance of fetal growth and birthweight standards should be evaluated in different ways, depending on whether they are intended for use in antenatal care or in epidemiological investigations.