The influence of birthweight on mortality and severe neonatal morbidity in late preterm and term infants: an Australian cohort study

Abstract

Background: The aim of this study was to detail incidence rates and relative risks for severe adverse perinatal outcomes by birthweight centile categories in a large Australian cohort of late preterm and term infants.

Methods: This was a retrospective cohort study of singleton infants (≥34⁺⁰ weeks gestation) between 2000 and 2018 in Queensland, Australia. Study outcomes were perinatal mortality, severe neurological morbidity, and other severe morbidity. Categorical outcomes were compared using Chi-squared tests. Continuous outcomes were compared using t-tests. Multinomial logistic regression investigated the effect of birthweight centile on study outcomes.

Findings: The final cohort comprised 991,042 infants. Perinatal mortality occurred in 1944 infants (0.19%). The incidence and risk of perinatal mortality increased as birthweight decreased, peaking for infants <1st centile (perinatal mortality rate 13.2/1000 births, adjusted Relative Risk Ratio (aRRR) of 12.96 (95% CI 10.14, 16.57) for stillbirth and aRRR 7.55 (95% CI 3.78, 15.08) for neonatal death). Severe neurological morbidity occurred in 7311 infants (0.74%), with the highest rate (19.6/1000 live births) in <1st centile cohort. There were 75,243 cases of severe morbidity (7.59% livebirths), with the peak incidence occurring in the <1st centile category (12.3% livebirths). The majority of adverse outcomes occurred in infants with birthweights between 10 and 90th centile. Almost 2 in 3 stillbirths, and approximately 3 in 4 cases of neonatal death, severe neurological morbidity or other severe morbidity occurred within this birthweight range.

Interpretation: Although the incidence and risk of perinatal mortality, severe neurological morbidity and severe morbidity increased at the extremes of birthweight centiles, the majority of these outcomes occurred in infants that were apparently "appropriately grown" (i.e., birthweight 10th-90th centile).

Funding: National Health and Medical Research Council, Mater Foundation, Royal Australian College of Obstetricians and Gynaecologists Women's Health Foundation - Norman Beischer Clinical Research Scholarship, Cerebral Palsy Alliance, University of Queensland Research Scholarship.

Keywords: Acidosis; Asphyxia; Birthweight centiles; Fetal growth restriction; Late preterm; Neonatal death; Neonatal morbidity; Neonatal mortality; Perinatal mortality; Placental dysfunction; Seizures; Small for gestational age; Stillbirth; Term.

Fig. 1

Study flow diagram—inclusion and exclusion criteria.

Fig. 2

Perinatal outcomes: a: Perinatal mortality, b: Severe neurological and other severe morbidity. c: Proportion of adverse outcomes by birth weight centile categories.

Fig. 3

The adjusted relative risk ratio and 95% of confidence interval of stillbirth, neonatal death, severe neurological morbidity and other severe morbidity across birthweight centile categories. aRRR—adjusted Relative Risk Ratio. Adjusted for lowest quintile of SEIFA, body mass index, nulliparity, drug use, maternal history of stillbirth, antepartum haemorrhage, diabetes mellitus and hypertension in pregnancy, and incorporating an interaction term for birthweight centile and method of birth.