Sometimes one breast might develop more quickly than the other, but most of the time they soon even out. With all this growing and developing going on, girls will notice an increase in body fat and occasional soreness under the nipples as the breasts start to enlarge - and that's normal. Gaining some weight is part of developing into a woman, and it's unhealthy for girls to go on a diet to try to stop this normal weight gain. If you ever have questions or concerns about your weight , talk it over with your doctor. It means that the puberty hormones have been doing their job.
Girls have two ovaries, and each ovary holds thousands of eggs.
Before the egg is released from the ovary, the uterus has been building up its lining with extra blood and tissue. If the egg gets to the uterus and is fertilized by a sperm cell, it will stay there and grow into a baby, using that extra blood and tissue to keep it healthy and protected as it's developing. Most of the time, though, the egg is only passing through. When the egg doesn't get fertilized and stay in the uterus, the uterus no longer needs the extra blood and tissue, so it leaves the body through the vagina as a menstrual period. A period usually lasts from 5 to 7 days, and about 2 weeks later a new egg is released and the whole cycle begins again.