Recently, Thomas et al. (2000) compared gestation-specific growth parameters derived from data on 27,229 neonates from 85 nurseries with parameters developed in the late 1960s. For neonates at < 30 weeks’ gestation, there were smaller variances and lower average weights, lengths, and head circumferences than previously published norms. For neonates > 36 weeks’ gestation, the variance was similar, but the neonates were larger and heavier. The authors concluded that using older growth curves resulted in misclassification of gender- and race-specific criteria for SGA and LGA. Since then, many investigators have observed an increase in birth weight at term (Orskou et al., 2001; Ananth and Wen, 2002; Surkan, 2004; Catalano et al., 2007). Hence, the use of current birth weight curves is important in the assessment of fetal growth. Oken et al. (2003) published U.S. birth weight curves based on the 1999 and 2000 United States Natality datasets from 22 through 44 weeks’ gestation.

Although gestational age is an important factor related to fetal growth, other factors affect not only fetal growth but also the pattern of growth. These include gender, with males growing more rapidly from the mid-third trimester through term (Figure 3-7); and maternal age, height, weight, GWG, obesity, and parity (Catalano et al., 2007). Paternal factors can also affect fetal growth, although they explain much less of the variance than maternal factors do (Klebanoff et al., 1998). High altitude results in decreased fetal growth, as does maternal hypoxia. Maternal medical problems, e.g., hypertensive disorders, autoimmune disease, and smoking can also result in decreased fetal growth. In contrast, maternal diabetes without evidence of vascular involvement often results in increased fetal growth (see Chapter 4 for detailed discussion).

The question of ethnic differences in fetal growth and implications for neonatal health has become more relevant recently. Kierans et al. (2008) evaluated all births in British Columbia from 1981 through 2000 and examined fetal growth and perinatal mortality in Chinese, South Asian, First Nation (Native American Indian), and other (primarily Caucasian) populations. They concluded that the ethnic differences in fetal growth rates were physiologic, not pathologic.

The rate of premature delivery (i.e., before 37 weeks’ gestation) in the United States is approximately 12.5 percent. As such, birth weight tables that rely on actual neonatal weights for preterm infants represent a much smaller percentage of all births. Furthermore, there is evidence that infants born prematurely are smaller than infants of the same gestational age who remain in utero (Weiner et al., 1985).

In summary, normal fetal growth is relatively uniform until mid-second trimester. At term there is much greater variation in fetal weight as a result of varying determinants of GWG and other maternal factors (see Chapter 4 for complete discussion). Lastly, there has been an increase in term birth

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