The remaining tissue in the human fetus is lean body mass or FFM, which consists primarily of glycogen, protein, and water. At birth the human fetus has approximately 40 g of glycogen, primarily in muscle and liver tissue (Girard and Ferre, 1982). The protein content of the term fetus is approximately 12.8 percent of total body weight, or 15 percent of FFM (i.e., about 500 g; Fomon et al., 1982; Spady, 1989). The remainder is water. In the human fetus at term, approximately 80 percent of FFM is water (Fomon et al., 1982).
With respect to temporal changes in fetal growth rates, generally the human fetus weighs approximately 1 kg at 28 weeks and then, over the next 12 weeks, gains approximately 2.5 kg. In the mid-second semester, fetal fat tissue begins to accrue and FFM as a percentage of total body weight begins decreasing. The gold standard for estimating fetal body composition is carcass analysis, although investigators have also used ultrasound to characterize the changes in composition that occur during gestation. Sparks (1984), reviewed data from 169 carcass analyses of fetuses and concluded that the differences in FFM are less variable than fat content at each gestational age. Changes in fetal FM may reflect changes in the intrauterine environment, while changes in FFM may be more representative of genetic factors. Bernstein et al. (1997) found that, although the rate of fetal FFM accretion appeared linear when considered in aggregate, the compartments of FFM changed differentially. Specifically, peripheral muscle growth accelerated and head circumference decelerated in late gestation; fetal fat deposition accelerated as a quadratic function. Hence, fetal growth of FM and FFM follow unique patterns and offer an additional means to assess normal and abnormal growth.
With respect to any observed association between neonate body composition and changes in maternal body composition, Butte et al. (2003) found that infant body composition at 2 weeks of age (FFM, FM, or percent FM) was not correlated with maternal body composition before or after pregnancy or with maternal gains in TBW, TBK, FFM, and FM during pregnancy. The investigators used DXA to assess body composition in 63 term singletons and related these changes to maternal body composition measured using a multi-component model. While neonate body composition bore no association with any other measured factor, birth weight correlated positively with prepregnancy weight (r = 0.34), prepregnancy FM (r = 0.32), GWG (r = 0.35), net GWG (r = 0.26), rate of weight gain (r = 0.28), gestational age (r = 0.49), gestational gains in TBW (r = 0.37), TBK (r = 0.35), and FFM (r = 0.39), but not FM. The investigators used multiple regression analysis to show that maternal FFM gains in the first, second, and third trimesters each independently contributed to birth weight, as did maternal TBW gains during the second and third trimesters and maternal TBK gain in the third trimester.