Figure 3
From: Theory of obstetrics: An epidemiologic framework for justifying medically indicated early delivery
Schematic depiction of pregnancy course and options for calculating the incidence of various perinatal phenomena. Schematic depiction of the course of several pregnancies illustrating the options for calculating the gestational age-specific rate (incidence) of stillbirth (Figure 3a), preeclampsia (Figure 3b), obstetric intervention (Figure 3c), and revealed small-for-gestational age (figure 3d). Figure 3a: Traditional calculation: Number of stillbirths at any gestational week/Number of total births at that gestational week = 1/4 = 250 per 1,000 total births in the first risk period and 1/5 = 200 per 1,000 total births in the second period. Fetuses at risk calculation: Number of stillbirths at any gestational week/Number of fetuses at risk of stillbirth at that gestational week = 1/16 = 63 per 1,000 fetuses at risk in the first risk period and 1/6 = 167 per 1,000 fetuses at risk in the second period. Figure 3b: Traditional calculation: Number of deliveries with preeclampsia at any gestational week/Number of deliveries at that gestational week = 1/4 = 250 per 1,000 deliveries for the first period and 1/5 = 200 per 1,000 deliveries for the second period.Fetuses at risk calculation: Number of new cases of preeclampsia at any gestational week/Number of pregnancies at risk of preeclampsia at that gestational week = 1/16 = 63 per 1,000 pregnancies at risk in the first period and 1/6 = 167 per 1,000 fetuses at risk in the second period. Figure 3c: Traditional calculation: Number of deliveries following labour induction or cesarean delivery at any gestational week/Number of deliveries at that gestational week = 2/4 = 500 per 1,000 deliveries for the first risk period and 3/5 = 600 per 1,000 deliveries for the second period. Fetuses at risk calculation: Number of deliveries following labour induction or cesarean delivery at any gestational week/Number of pregnancies at risk of labour induction or cesarean delivery at that gestational week = 2/16 = 125 per 1,000 pregnancies at risk for the first period and 3/6 = 500 per 1,000 pregnancies at risk for the second period.Figure 3d: Traditional calculation: SGA rate assumed to be uniform 10% or 3% at each gestation depending on cutoff used (10th percentile or 3rd percentile). Fetuses at risk calculation: Number of new SGA cases at any gestational week/Number of fetuses at risk of SGA at that gestational week = 1/15 = 67 per 1,000 fetuses at risk for the first risk period and 1/4 = 250 per 1,000 fetuses at risk for the second risk period. Fetuses at risk calculation for revealed SGA rate: Number of revealed SGA cases at any gestational week/Number of fetuses at risk of SGA birth at that gestational week = 2/16 = 125 per 1,000 fetuses at risk in the first risk period and 2/6 = 333 per 1,000 fetuses at risk in the second period.