Should Fetal Growth Charts Be References or Standards?

BACKGROUND: Fetal growth standards (prescriptive charts derived from low-risk pregnancies) are theoretically better tools to monitor fetal growth than conventional references. We examined how modifying chart inclusion criteria influenced the resulting curves. METHODS: We summarized estimated fetal weight (EFW) distributions from a hospital’s routine 32-week ultrasound in all nonanomalous singleton fetuses (reference) and in those without maternal–fetal conditions affecting fetal growth (standard). We calculated EFWs for the 3rd, 5th, 10th, and 50th percentiles, and the proportion of fetuses each chart classified as small for gestational age. RESULTS: Of 2309 fetuses in our reference, 690 (30%) met the standard’s inclusion criteria. There were no meaningful differences between the EFW distributions of the reference and standard curves (50th percentile: 1989 g reference vs. 1968 g standard; 10th percentile: 1711 g reference vs. 1710 g standard), or the proportion of small for gestational age fetuses (both 9.9%). CONCLUSIONS: In our study, there was little practical difference between a fetal growth reference and standard for detecting small infants.