Detecting small for gestational age infants: the development of a population-based reference for Washington state.

Small for gestational age (SGA) infants are at increased risk for morbidity and mortality. The objective of this study was to develop a birthweight for gestational age reference that more accurately represents the Washington state population, focusing on SGA. Washington state birth certificate files of singleton births in 1989 to 1998 were used to develop the 3rd, 10th, 50th, and 90th percentiles of birthweight for gestational age for males and females. The Washington state 10th percentile curve most closely approximates a nationally representative reference, whereas an earlier but widely used California-based reference had the lowest centiles across gestational age with few exceptions. Using the Washington reference, 8.4% of Washington births would be classified as SGA (<10th percentile), compared with 5.5 and 7.4% using the California and national reference, respectively. The new reference may be helpful in assessing local regional data, and other areas with similar demographics, and provide more relevant clinical guidance.