Fetal growth standards: The NICHD Fetal Growth Study Approach in Context with INTERGROWTH-21st and the World Health Organization Multicentre Growth Reference Study

摘要

Three recently completed longitudinal cohort studies have developed intrauterine fetal growth charts, one in the United States and two international. This expert review compares and contrasts the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Fetal Growth Studies, INTERGROWTH-21^st (INTERGROWTH) and World Health Organization Multicentre Growth Reference Study (WHO Fetal) conclusions in light of differences in aims, sampling frames, and analytical approaches.An area of controversy is whether a single growth reference is representative of growth regardless of ethnic or country origin. The INTERGROWTH and WHO Fetal studies used a similar approach as the WHO Multicentre Growth Reference Study (WHO MGRS) for infants and children, the aim of which was to create a single international reference for the best physiologic growth for children ages 0–5 years. INTERGROWTH made the same assumption, i.e., that there would be no differences internationally among countries or racial/ethnic groups in fetal growth when conditions were optimal. INTERGROWTH found differences in crown-rump length and head circumference among countries, but interpreted the differences as not meaningful and presented a pooled standard. WHO Fetal was designed to create a pooled reference, although they evaluated for and presented country differences, along with discussion of the implications. The NICHD Study was designed to assess whether racial/ethnic-specific fetal growth standards were needed, in recognition of the fact that fetal size is commonly estimated from dimensions (head circumference, abdominal circumference, and femur length) where there are known differences in children and adults of differing racial/ethnic groups. A pooled standard would be derived if no racial/ethnic differences were found. Highly statistically significant racial/ethnic differences in fetal growth were found resulting in the publication of racial/ethnic-specific derived standards.Despite all three studies including low-risk status women, the percentiles for fetal dimensions and estimated fetal weight (EFW) varied among the studies. Specifically, at 39 weeks, the 50^th percentile for EFW was 3502 g for whites, 3330 g for Hispanics, 3263 g for Asians and 3256 for blacks in the NICHD Study, compared with 3186 g for INTERGROWTH and 3403 g for WHO Fetal. When applying these standards to a clinical population, it is important to be aware that different percentages of small- (SGA) and large-for-gestational age (LGA) fetuses will be identified. Also, it may be necessary to use more restrictive cut-points, such as the 2.5^th or 97.5^th, for SGA or LGA, respectively. Ideally, a comparison of diagnostic accuracy, or misclassification rates, of SGA and LGA in relation to morbidity and mortality using different criteria is necessary to make recommendations, and remains an important data gap. Identification of the appropriate percentile cut-offs in relation to neonatal morbidity and mortality is needed in local populations depending on which fetal growth chart is used. On a final point, assessment of fetal growth with a one-time measurement remains standard clinical practice, despite recognition that a single measurement can only indicate size. Ultimately, it is knowledge about fetal growth in addition to other factors and clinical judgment that should trigger intervention.