RÉSUMÉ
Background: Adolescent is a special period in life with the transition from childhood to adulthood. At this age period, adolescent experience physical and psychological changes with typical characteristics of puberty is the first ejaculation in male and first menstrual cycle in female. Objective: To explore the onset of puberty and knowledge on pubertal changes, sexuality and contraception in adolescent and various related factors. Subject and methods: A longitudinal study of adolescent health is conducted on 12.447 subjects (including 6.108 males, 6.339 females), living in Chi Linh district, Hai Duong province. Data used in this article is from the baseline survey of the adolescent health research project \u2013 a field site of the Hanoi School of Public Health. Results and Conclusion: The pubertal age is 15.6 years for male and 14.6 years for female. The onset of puberty in girls is earlier than that in their boy counterpart and is earlier in younger age groups, that suggest the trend of earlier onset of puberty in adolescent. The pubertal age is earlier in urban adolescent compared to the rural ones and is earlier among those having better economical conditions compared to poorer ones. The difference is statistically significant (p<0.05). Adolescent lack of knowledge on puberty, sexuality and contraception. 57.8% of adolescent knows at least one of the pubertal characteristics and only 6.5% of males and 17.6% of females knows the time most fertile in the female menstrual cycle. 49.6% of adolescents have heard about contraception, in which 56% of them (53% males and 58% females) heard about condoms. In general, the knowledge on puberty, sexuality and contraception is higher in female, urban adolescent, higher education and older adolescent. The difference is statistically significant (p<0.05 or <0.001).