Sex, birth weight, and the risk of stillbirth in Scotland, 1980-1996.

The aim of this study was to determine whether the risk of stillbirth associated with male fetal sex was modified by fetal growth. The study group consisted of all singleton first births weighing greater than 500 g delivered between 28 and 43 weeks gestation in Scotland in 1980-1996 (n = 469,152). Overall, male fetuses were at an increased risk of stillbirth (relative risk = 1.19, 95 percent confidence interval: 1.10, 1.29). There was a significant negative interaction between male sex and increasing birth weight quintile in term, but not preterm, births. The interaction was virtually identical when calculated independently for births in the periods 1980-1987 and 1988-1996. There were linear decreases in the proportion of stillbirths and the proportion of birth weights in the lowest quintile over the period 1980-1996. Adjustment for year of birth did not affect the relation between male sex and stillbirth. However, adjustment for birth weight resulted in a loss of the association between year of birth and risk of stillbirth. The authors concluded that 1) the association between male sex and stillbirth diminishes with increasing birth weight quintile, and 2) there was a fall in the proportion of stillbirths in Scotland between 1980 and 1996, which may have been due to a fall in the proportion of small babies over the same period.