DIFFERENTIAL GROWTH OF FETAL TISSUES DURING THE 2ND HALF OF PREGNANCY

OBJECTIVE: Our purpose was to examine the pattern of growth of both fe tal lean body mass incorporating bone, brain, and muscle and subcutane ous fat mass during the course of normal pregnancy. We hypothesized th at there are detectable differences in the accretion of fat versus lea n body mass. STUDY DESIGN: To establish our method we correlated stand ardized cross-sectional ultrasonographic images of the fetal extremiti es with anthropometric assessment of neonatal body composition in 25 s ubjects. Subsequently 36 nonsmoking women with normal prepregnancy bod y mass index, normal glucose screening results, and no medical or obst etric complications were recruited. We performed 135 ultrasonographic examinations between 19 and 40 weeks' gestation (mean 3.8 scans per fe tus, range 2 to 6) at 4-week intervals. Lean body mass measures includ ed biparietal diameter, head circumference, and femur length. Fetal su bcutaneous 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 weig ht distribution. Stepwise regression analysis established best-fit equ ations for fetal measurements obtained ultrasonographically. Independe nt variables included gestational age, maternal age, weight gain in pr egnancy, parity, fetal gender, and maternal prepregnancy weight. RESUL TS: Fetal bone growth was best described by a second-order quadratic e quation demonstrating deceleration with advancing gestational age (p < 0.0001, R(2) 0.92 to 0.96). A quadratic equation that accelerates wit h advancing gestation best described lean body mass accretion in the e xtremities (p < 0.0001, R(2) = 0.85 to 0.86). Fetal fat deposition in the extremities was characterized by an accelerating quadratic equatio n when plotted against gestational age with maternal age and prepregna ncy 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 resu lt of an accelerated rate of growth in late gestation, the measurement of fetal fat will provide a more sensitive and specific marker of abn ormal fetal growth when compared with index values of lean body mass.