Differential growth of fetal tissues during the second half of pregnancy

OBJECTIVE: Our purpose was to examine the pattern of growth of both fetal lean body mass incorporating bone, brain, and muscle and subcutaneous fat mass during the course of normal pregnancy. We hypothesized that there are detectable differences in the accretion of fat versus lean body mass. STUDY DESIGN: To establish our method we correlated standardized cross-sectional ultrasonographic images of the fetal extremities with anthropometric assessment of neonatal body composition in 25 subjects. Subsequently 36 nonsmoking women with normal prepregnancy body mass index, normal glucose screening results, and no medical or obstetric complications were recruited. We performed 135 ultrasonographic examinations between 19 and 40 weeks' gestation (mean 3.8 scans per fetus, range 2 to 6) at 4-week intervals. Lean body mass measures included biparietal diameter, head circumference, and femur length. Fetal subcutaneous fat and lean body mass were examined both in the mid upper arm and midthigh by standardized cross-sectional images. All neonates were born between 37 and 42 weeks' gestation and had normal birth weight distribution. Stepwise regression analysis established best-fit equations for fetal measurements obtained ultrasonographically. Independent variables included gestational age, maternal age, weight gain in pregnancy, parity, fetal gender, and maternal prepregnancy weight. RESULTS: Fetal bone growth was best described by a second-order quadratic equation demonstrating deceleration with advancing gestational age (p < 0.0001, R(2) 0.92 to 0.96). A quadratic equation that accelerates with advancing gestation best described lean body mass accretion in the extremities (p < 0.0001, R(2) = 0.85 to 0.86). Fetal fat deposition in the extremities was characterized by an accelerating quadratic equation when plotted against gestational age with maternal age and prepregnancy weight contributing significantly (p < 0.0001, R(2) = 0.80 to 0.81). CONCLUSION: Consistent with our hypothesis, fetal fat and lean body mass demonstrate unique growth profiles. We speculate that, as a result of an accelerated rate of growth in late gestation, the measurement of fetal fat will provide a more sensitive and specific marker of abnormal fetal growth when compared with index values of lean body mass.