Perinatal outcome in SGA births defined by customised versus population-based birthweight standards

Objective To determine whether customised birthweight standard improves the definition of small for gesta- ga tional age and its association with adve rse pregnancy outcomes such as stillbirth, neonatal death, or low Apgar sco re. Design Population based cohort study. Population Births in Sweden between 1992-95 (n = 326,377). Methods Risks of stillbirth, neonatal death, and Apgar score under four at five minutes were calculated for the lowest 10% birthweights according to p opulation-based and customised standards, and were compared with the data f rom the group with birthweights over this limit. Population attributable ri sks for stillbirth using various birthweight centile cutoffs were calculate d for the two standards. Outcome measures Odds ratios and 95% confidence intervals for stillbirth, n eonatal death and Apgar score under four at five minutes, and population at tributable risks for stillbirth at different birthweight centiles. Results Risks of stillbirth, neonatal death, and Apgar score under four at five minutes and population attributable risks of stillbirth were consisten tly higher if 'small for gestational age' was classified by a customised ra ther than by the population-based birthweight standard. Compared with infan ts who were not small for gestational age by both standards, the odds ratio for stillbirth was 6.1 (95% CI 5.0-7.5) for small for gestational age by c ustomised standard only, whereas it was L2 (95 % CI 0.8-1.9) for small for gestational age by population standard only. Conclusions Compared with the population-based birthweight standard, a cust omised birthweight standard increases identification of fetuses at risk of stillbirth, neonatal death and Apgar score under 4 at 5 minutes, probably d ue. to improved identification of fetal growth restriction.