Disorders of fetal growth occur at both ends of the spectrum, either fetal-growth restriction or macrosomia. Each poses concern because of associated morbidities and potential mortality. However, in both categories most of these newborns are ultimately deemed normal and healthy but merely constitutionally small or large. The clinical challenge thus lies in the evaluation and management of suspected fetal-growth disorders.
Fetal growth may be divided into three phases. The initial phase of hyperplasia occurs in the first 16 weeks and is characterized by a rapid rise in cell number. The second phase, which extends up to 32 weeks’ gestation, includes both cellular hyperplasia and hypertrophy. After 32 weeks, fetal mass accrues by cellular hypertrophy, and it is during this phase that most fetal fat and glycogen accumulate. The corresponding fetal-growth rates during these three phases approximate 5 g/d at 15 weeks’ gestation, 15 g/d at 24 weeks’, and 30 g/d at 34 weeks’ (Grantz, 2018; Williams, 1982).
In the National Institute of Child Health and Human Development Fetal Growth Studies, serial sonographic evaluations were performed in 1733 nonobese, low-risk pregnancies at 12 sites across the United States (Grantz, 2018). As shown in Figure 47-1, growth velocity peaked at 35 weeks’ gestation (Grantz, 2018). The investigators demonstrated that fetal growth varies considerably and that it is not highly correlated with fetal birthweight percentile. Some fetuses with initial estimated weights below the 5th percentile maintained their growth velocity and ultimately weighed more at birth than other fetuses whose weight percentiles were initially higher but whose growth was slower. Such findings support our understanding that large and small fetal weight percentiles reflect constitutional size in some but indicate disordered growth in others. They further highlight the importance of serial sonography when abnormal growth is a concern.
Fetal weight gain in grams per day by race/ethnicity throughout gestation.
Accurate gestational age assessment is critical for determining whether birthweight is normal. Current normative data are based on birthweights from pregnancies in which gestational age is established using an obstetrical estimate that includes sonography and discussed in Chapter 14 (p. 248) (American College of Obstetricians and Gynecologists, 2019b). The birthweight percentiles shown in Table 47-1 were derived using data from more than 3 million liveborn singletons delivered across the United States in 2011 (Duryea, 2014). As shown in Figure 47-2, use of a birthweight percentile curve in which gestational age is based on a last menstrual period alone yields significantly larger weights for a given gestational age, particularly in the preterm period. The accuracy of a birthweight reference thus has potential to affect the prevalence of neonates diagnosed as small or large for gestational age.