Small-for-gestational-age infants classified by customized or population birthweight centiles: impact of gestational age at delivery

OBJECTIVE: Evidence of placental disease and poor perinatal outcome is more common in infants who are small by customized centiles, compared with population centiles. Because preterm births are more likely to be associated with placental pathology, a greater proportion of preterm births are likely to be small for gestational age (SGA) by customized centiles, compared with population centiles. Our objective was to compare the proportion of infants classified as SGA by customized and population birthweight centiles at different gestational ages at delivery. STUDY DESIGN: This was a retrospective observational study of 17,855 nulliparous women delivering between 1992 and 1999 at National Women's Hospital, Auckland, New Zealand. The proportion of SGA infants (birthweight less than the 10th centile) classified by customized and population birthweight centiles delivering at less than 34 weeks, 34-36(+6) weeks, and 37 weeks or longer were compared. RESULTS: A total of 1847 infants (10.3%) were customized SGA, compared with 2111 (11.8%) who were population SGA ( relative risk [RR] 0.9, 95% confidence interval [CI] 0.8 to 0.9). Of preterm deliveries less than 34 weeks (n = 392), 29.1% were customized SGA and 17.1% were population SGA (RR 1.7, 95% CI 1.3 to 2.2). Of deliveries at 34-36(+6) weeks (n = 946), 18.0% were customized SGA and 13.7% were population SGA ( RR 1.3, 95% CI 1.1 to 1.6). The converse was observed at term (n = 16,517), 9.5% classified as customized SGA and 11.5% as population SGA ( RR 0.82, 95% CI 0.77 to 0.87). Of all early preterm perinatal deaths ( less than 34 weeks) 31 of 72 infants (43%) were customized SGA and 23 of 72 infants (32%) were population SGA. There were no perinatal deaths or deliveries less than 34 weeks in infants who were classified as SGA by population criteria only. CONCLUSION: Customized centiles classified more infants as SGA, compared with population centiles, in preterm births but not for term births in nulliparous women.