Effect of sex and gestational age on neonatal body composition.

To determine the effects of length of gestation and sex on infant body composition, air displacement plethysmography was performed in forty-six full-term neonates at 3 d of life and during the week prior to hospital discharge in 180 preterm neonates. Fat mass, as a percentage of body weight, was higher in preterm than in term infants (13.4 (SD 4.2) v. 10.1 (sd 3.7) %, respectively; P= 0.001). The absolute amount of fat mass did not differ between preterm and full-term newborns (323 (SD 126) v. 335 (SD 138) g; P= 0.58), whereas lean body mass was lower in preterm than in term infants (2055 (SD 280) v. 2937 (SD 259) g, respectively; P< 0.001). Among full-term infants, fat mass was higher in females than in males (11.1 (SD 3.7) v. 9.0 (SD 3.3) %, respectively; P= 0.047), whereas we did not observe any sex difference in preterm infants (13.5 (SD 4.1) v. 13.4 (SD 4.3) %; P= 0.89). Our data suggest that by the time they are discharged from hospital: (1) preterm infants have a higher percentage of body fat than term neonates and (2) this is presumably due to a lesser accretion in lean body mass in the first few weeks of extra-uterine life, particularly in boys.