A parsimonious explanation for intersecting perinatal mortality curves: understanding the effect of plurality and of parity

Abstract

BACKGROUND: Birth weight- and gestational age-specific perinatal mortality curves intersect when compared across categories of maternal smoking, plurality, race and other factors. No simple explanation exists for this paradoxical observation. METHODS: We used data on all live births, stillbirths and infant deaths in Canada (1991-1997) to compare perinatal mortality rates among singleton and twin births, and among singleton births to nulliparous and parous women. Birth weight- and gestational age-specific perinatal mortality rates were first calculated by dividing the number of perinatal deaths at any given birth weight or gestational age by the number of total births at that birth weight or gestational age (conventional calculation). Gestational age-specific perinatal mortality rates were also calculated using the number of fetuses at risk of perinatal death at any given gestational age. RESULTS: Conventional perinatal mortality rates among twin births were lower than those among singletons at lower birth weights and earlier gestation ages, while the reverse was true at higher birth weights and later gestational ages. When perinatal mortality rates were based on fetuses at risk, however, twin births had consistently higher mortality rates than singletons at all gestational ages. A similar pattern emerged in contrasts of gestational age-specific perinatal mortality among singleton births to nulliparous and parous women. Increases in gestational age-specific rates of growth-restriction with advancing gestational age presaged rising rates of gestational age-specific perinatal mortality in both contrasts. CONCLUSIONS: The proper conceptualization of perinatal risk eliminates the mortality crossover paradox and provides new insights into perinatal health issues.

Figure 1

Gestational age, birth weight and birth weight-specific perinatal mortality among singletons and twins. Legend text: Gestational age (upper panel) and birth weight (middle panel) distributions and birth weight-specific perinatal mortality rates (lower panel) among singleton and twin births, Canada (excluding Ontario), 1991–97.

Figure 2

Stillbirth, early neonatal death and perinatal mortality rates among singleton and twin births (per convention). Legend text: Gestational age-specific stillbirth (upper panel), early neonatal death (middle panel) and perinatal mortality (lower panel) rates among singleton and twin births calculated as per convention (i.e., per 1,000 total births/live births at each gestation), Canada (excluding Ontario), 1991–97.

Figure 3

Stillbirth, early neonatal death and perinatal mortality rates among singleton and twin births (fetuses-at-risk approach). Legend text: Gestational age-specific stillbirth (upper panel), early neonatal death (middle panel) and perinatal mortality (lower panel) rates among singleton and twin births calculated using the fetuses-at-risk approach, Canada (excluding Ontario), 1991–97.

Figure 4

Gestational age and birth weight distributions and perinatal mortality rates among nulliparous and parous women. Legend text: Gestational age (upper left) and birth weight (upper right) distributions and gestational age-specific perinatal mortality rates, calculated as per conventional (lower left) and using the fetuses-at-risk approach (lower right), among births to nulliparous vs parous women, Canada (excluding Ontario), 1991–97.

Figure 5

Birth, fetal growth-restriction and perinatal mortality rates among singletons vs twins and nulliparous vs parous women. Legend text: Gestational age-specific birth rates (left) and fetal growth-restriction rates (right) in relation to perinatal mortality rates among singletons vs twin births (upper) and among births to nulliparous vs parous women (lower), Canada (excluding Ontario) 1991–97.