The curves by Alexander (1996) and Duryea (2014) and their associates are most accurately termed a population rerence, rather than a standard. A population reference incorporates pregnancies of varying risks, along with the resulting outcomes, both normal and abnormal. In contrast, a stanard incorporates normal 1414 1573 2053 2579 2861 pregnancies with normal outcomes. Because population refer 34 1632 1793 2296 2846 3093 ences include preterm births, which are more likely to be growth 1871 2030 2549 3119 3345 restricted, it has been argued that the associated birthweight data 36 2117 2270 2797 3380 3594 overestimate deficient fetal growth (Mayer, 2013; Zhang, 2010).
37 2353 2500 3025 361n2 381n8 One recent project sought to define regional standards in health and socioeconomic conditions. Growth trajectories from the International Fetal and Newborn Growth Consortium for these curves are based on speciic ethnic or regional groups, they do not represent the entire population.
To address this, birthweights such as those shown in Table 4-1 are derived nationwide in the United States. Shown in Figure 4-2 are growth curves rom more thn 3.2 million mothers with singleton livebon neonates in the United States during 1991 and 2011 5,000 -By last menstrual period 4,000 impaired growth. hus, they cannot be used to identiy the fetus that fails to achieve an expected size but whose birthweight is above the 21st Century-INTERGROWTH 21 were similar in these eight: China, India, Kenya, Brazil, Oman, Italy, the United Kingdom, and the United States (Villar, 2014). However, an international standard based on the healthiest women is of questionable value (Hanson, 2015).
Most of what is known regarding normal and abnormal human fetal growth is actually based on birthweights that are assembled as references for fetal growth at particular gestational ages.
his is problematic, however, because birth weight does not deine the rate of fetal growth.
Indeed, such birthweight curves reveal com- the of � the 10th percentile. For example, a fetus with 3,000 ) a birthweight in the 40th percentile may not have achieved its genomic growth potential for a birthweight in the 80th percentile.
: 2,000 m The rate or velociy of fetal growth can be estimated by serial sonographic anthropometry. For example, Milovanovic (2012) 1,000 demonstrated that the growth rate of intrin borns (those below the 10th percentile) 22 24 26 28 30 32 34 36 38 40 42 44 approximates that of appropriate-for-gestaGestational age (weeks) tional-age neonates. However, diminished growth velocity may be linked to perinatal
FIGURE 44-2 Fetal-growth curves for births in the United States in 2011. Curves vary depending on whether gestational age was calculated from the last menstrual changes that are independent of birthweight.