Teenage pregnancy has been associated with adverse effects for the mother and the newborn (NB). In order to compare body composition (BC) between adolescents (Ad) and mature women (MW) during pregnancy and to determine the difference in birthweight and perinatal morbidity, pregnant Ad (n = 40) and MW (n = 227) were studied. BC changes between the second and third trimesters were determined by multifrequency bioelectrical impedance analysis, and birthweight and NB morbidity were evaluated. During the second and third trimesters of the pregnancy, fat mass was lower in the Ad group [16 kg (13–19)] than in the MW group [22 kg (17–27)] (P < 0.01; median and quartiles 1–3). Fat-free mass increased by 3.09 kg (2.29–4.20) and 2.20 kg (1.0–3.59) (P ≤ 0.01), and total body water increased by 2.77 L (0.84–4.49) vs. 2.04 L (0.55–3.89) (P = 0.36), in the Ad and MW groups, respectively (median and quartiles 1–3). Birthweight was not significantly different between NBs of Ad (3223 ± 399 g) and NBs of MW (3312 ± 427 g, P = 0.22). The youngest Ad (<18 year old, n = 8) had NB with lower birthweight than MW (3031 ± 503 g, P = 0.06). NBs of Ad mothers showed a non-significant trend towards a higher rate of morbidity relative to the NBs of MW. In conclusion, the BC of Ad differs from that of MW during pregnancy. In addition, the NB infants of Ad mothers tended to have a lower birthweight than those from MW, a result that suggests that the Ad should be in strict prenatal control.