A new study finds that girls in the US are reaching puberty(思春期)at earlier ages than they have in the past. The analysis adds to a growing body of evidence suggesting that the onset(開始)of puberty in girls is shifting earlier. Experts cannot say for certain what is behind the shift, but several factors are likely to blame, including increased rates of obesity and environmental chemicals that mimic the effects of estrogen.
Furthermore, girls who develop early often become sexually active earlier than those who develop later and have a greater risk of depression.