Background
Detection of abnormal fetal growth is vital to antenatal care, and traditionally birthweights that are <10th or >90th centile are classified as small or large for gestational age (LGA). Evidence regarding outcomes for birthweight centiles outside these extremes remains unclear.

Aims
To evaluate the relationship between birthweight centile and perinatal death and determine the ‘optimum’ birthweight centile with the lowest rate of perinatal mortality.

Method
Data on all Victorian births from 1999 to 2008 were stratified into smaller subsets than the traditional small for gestational age (SGA) (<10th centile), appropriate for gestational age (AGA) (10–90th centile) and LGA (>90th centile) and analysed by all gestations, for term births alone, and using the ‘fetus at risk’ approach. Multiple logistic regression was used to adjust for age, parity and co‐morbidities.

Results
For term births, the ‘optimum’ birthweight centile was the 50–90th range (1.1 perinatal deaths/1000 births). Lower birthweight centiles had significantly higher rates of perinatal death – even those that would be classified as AGA. Babies with a 10–25th birthweight centile had a two‐fold increased risk of perinatal death (AOR 2.10, 95% CI 1.6, 2.7). Even those with a 25–50th birthweight centile had higher perinatal mortality rates (AOR 1.58, 95% CI 1.3, 2.0). There was no strong evidence of higher perinatal mortality in larger birthweight centiles, except term births >99th centile. The ‘fetus at risk’ analysis showed a rise in perinatal mortality after 37 weeks’ gestation for all birthweight centiles, particularly for SGA babies.

Conclusion
Babies with a birthweight below the 50th centile are at greater risk of perinatal mortality compared with the ‘optimum’ ≥50 to <90th centile group.