Reduced birthweight in short or primiparous mothers: Physiological or pathological?

Objective Customisation of birthweight-for-gestational-age standards for maternal characteristics assumes that variation in birth weight as a result of those characteristics is physiological, rather than pathological. Maternal height and parity are among the characteristics widely assumed to be physiological. Our objective was to test that assumption by using an association with perinatal mortality as evidence of a pathological effect. Design Population-based cohort study. Setting Sweden. Population A total of 952 630 singletons born at ≥28 weeks of gestation in the period 1992-2001. Methods We compared perinatal mortality among mothers of short stature (<160 cm) versus those of normal height (≥160 cm), and primiparous versus multiparous mothers, using an internal reference of estimated fetal weight for gestational age. The total effects of maternal height and parity were estimated, as well as the effects of height and parity independent of birthweight (controlled direct effects). All analyses were based on fetuses at risk, using marginal structural Cox models for the estimation of total and controlled direct effects. Main outcome measures Perinatal mortality, stillbirth, and early neonatal mortality. Results The estimated total effect (HR; 95% CI) of short stature on perinatal death among short mothers was 1.2 (95% CI 1.1-1.3) compared with women of normal height; the effect of short stature independent of birthweight (controlled direct effect) was 0.8 (95% CI 0.6-1.0) among small-for-gestational-age (SGA) births, but 1.1 (95% CI 1.0-1.3) among non-SGA births. Similar results were observed for primiparous mothers. Conclusions The effect of maternal short stature or primiparity on perinatal mortality is partly mediated through SGA birth. Thus, birthweight differences resulting from these maternal characteristics appear not only to be physiological, but also to have an important pathological component.