It has been suggested that TSF thickness of adolescent gravidas is related to birth weight of their infants, especially in pregnancies of younger adolescents. We hypothesized that the effect of TSF changes between the beginning and end of the 3rd trimester on birth weight in full term gestation, would be different for younger compared to older Caucasian adolescents. Serial TSF measures of subjects (≤18 yr) retrospectively studied at three U.S. university adolescent prenatal clinics were analyzed. 64 subjects who delivered ≥37 wk and for whom 24 and 37 wk measures were available were included (mean age 16.2 yr). Subjects were divided into younger (Y) and older(O) groups, ≤16 yr vs. >16 yr. Median birth weights were not significantly different (Y=3455 g, O=3300 g). Relationships of TSF change, and values at 24 and 37 wk (along with other maternal characteristics), to birth weight, was tested by regression analysis. Birth weight was inversely related to TSF change in the total sample with a correlation of -0.24. Theoretically, a decrease in TSF represented a mobilization of stored energy during the 3rd trimester resulting in increased birth weight. For Y, TSF change (p=0.01),(R2=0.18), was significantly related, and both wk 24 (p=0.02) and wk 37 TSF (p=0.01) were independently and significantly related to birth weight(R2=0.20). For O, final weight (p=0.01), age at menses (p=0.02) and wk 24 TSF (p=0.04) were significantly related to birth weight (R2=0.36); TSF change was not. We conclude that TSF thickness of adolescent gravidas, especially a decrease in the 3rd trimester, influences birth weight; the effect is greater for infants of mothers 16 yr and younger.