Section 1, Article 2 - For girls, one change brought on by puberty is the first menstrual period, known as menarche. This indicates that ovulation has begun and pregnancy is now biologically possible. However, ovulation and menstruation are usually irregular for years to come. The onset of menstruation greatly varies from country to country; girls from affluent countries begin menstruation much earlier than girls from poorer countries. General health also seems to be a factor; better-nourished and healthier girls tend to start earlier than girls with malnourishment or chronic diseases. Moreover, body fat percentage may also be a factor; girls with higher body fat start earlier than leaner, athletic girls or girls with low body fat ().