Figure 5.

Illustration of the bias possibly occurring in studies of environmental effects on fetal growth, when gestational age is defined by early ultrasound measures (96). Hypothetical evolution of a fetal measurement (e.g., fetal length) during pregnancy, for a pregnancy exposed or unexposed to an environmental factor affecting fetal growth from early pregnancy. The ultrasound examination leads the obstetrician to correct the date of conception of the exposed pregnancy by Δt, so this exposed pregnancy is not compared with unexposed pregnancies with the same gestational age D (solid curve) as should be the case but with gestational age D − Δt (dashed curve). Consequently, the estimated difference in the gestational age–specific fetal measurement at birth between exposed and unexposed pregnancies is not the correct value β but a smaller value β′.