As with fetal growth patterns, multiple factors are associated with alterations in fetal body composition, including:
genetic (e.g., at birth, male fetuses have greater lean body mass than females, and as a consequence, females have a higher percentage of body fat (Catalano et al., 1995; Ibanez et al., 2008);
maternal parity, which is positively correlated with neonatal adiposity (Harvey et al., 2007);
prepregnancy BMI, with birth weight significantly greater in neonates of overweight and obese women than underweight or normal weight women because of increased FM, not FFM (Sewell et al., 2006; Hull et al., 2008);
maternal weight gain, which is associated with both increased fetal FFM and increased FM (and maternal pregravid BMI [Catalano and Ehrenberg, 2006]);
maternal medical problems, such as gestational diabetes mellitus (GDM), that are associated with an increase in birth weight (again because of increased FM, and in the macrosomic neonate a relative decrease in FFM) (Catalano et al., 2003; Durnwald et al., 2004);
environmental factors (see Chapter 4) such as maternal smoking which has a negative effect on fetal growth on the order of 150 g, which primarily decreases fetal FFM (Lindsay et al., 1997); and
increased altitude, which has been reported to be associated with a 339-g decrease in birth weight (Ballew and Haas, 1986, showed that crown-head length was reduced by 1 cm, although the sum of five skinfolds was 5 mm greater, in those born at high altitude compared to those born at sea level).
In their study of > 400 newborns using total body electrical conductivity, Catalano and Ehrenberg (2006) found that maternal pregravid BMI had a stronger correlation with fetal adiposity than maternal weight gain and GDM did.
In summary, the human fetus has a high percentage of body fat (12-16 percent) at birth compared to most mammalian species. Fetal fat mass contributes the greatest percentage of variance in birth weight, is affected by the in utero environment, and is more strongly correlated with maternal pregravid BMI than GWG.
The committee reviewed evidence on amniotic fluid volumetric changes in gestation and determined that amniotic fluid is an important component of GWG. There are four major sources of volume flow into and out of