The risk of neonatal death in relation to birth weight and maternal hypertensive disease in infants born at 24-32 weeks

Abstract

Objective: To determine the risk of neonatal death (NND) in relation to birth weight for gestational age and the presence or absence of maternal hypertensive disease in preterm neonates.

Design: Record linkage of maternity data and neonatal mortality data.

Setting: Scotland, UK.

Population: A group of 6946 live singleton preterm neonates without lethal congenital abnormalities born at 24-32 weeks between 1986 and 1992 inclusive. This group included 1448 cases of maternal hypertensive disease and 850 neonatal deaths.

Main outcome measure: Neonatal death.

Results: The median birth weight for each gestational week was estimated from a fitted curve and each birth weight was recalculated as a multiple of the relevant median. The frequency of NND was much higher at lower gestations (73% at 24 weeks down to 2% at 32 weeks). Though the overall frequency of NND was lower in cases with hypertensive disease (8.6% versus 13.2%) this can be attributed to the fact that there were relatively fewer hypertensive cases in the high risk group at 24-27 weeks. In the 5498 cases not associated with maternal hypertensive disease, there were 726 NNDs. The mean MoM of birthweight for these NNDs was 0.982 (95% CI 0.967-0.996); this was only marginally different from the population mean (0.998; 95% CI 0.993-1.004). In the 1448 cases with maternal hypertensive disease, there were 124 NNDs. The overall birthweight for gestational age in the hypertensive group was substantially less than that of the whole population (mean MoM 0.84; 95% CI 0.83-0.85) and that of the 124 NNDs was still lower (mean MoM 0.75; 95% CI 0.724-0.782). For both hypertensive and non-hypertensive cases, inspection of the data categorised into deciles showed that there was a continuous increase in the frequency of NND throughout the weight range, being lowest for the heaviest babies and highest for those in the lower centiles.

Conclusion: (1) There is a relationship between birthweight for gestational age and risk of NND in infants born at 24-32 weeks; (2) this relationship is a continuum throughout the whole range of birthweight, not focused exclusively on a group defined as SGA; (3) provided appropriate birthweight standards are used, there is no extra effect on mortality from maternal hypertensive disease.