Most girls weigh about 105 pounds (that’s seven and a half stone or just under 48 Kg) and are 17% fat when their periods start. This is because in our evolutionary history (and today) a female must have adequate fat deposits to carry a pregnancy and nourish a growing baby. That said teenage mothers tend to have smaller babies and be at higher risk of premature labour than older mothers.
Low body fat levels are the reason that very underweight girls and girls with anorexia nervosa have delayed puberty or their periods stop.
Fat deposits stimulate the onset of puberty via a hormone called leptin produced by fat cells. Leptin is released into the bloodstream and travels to the brain. When the circulating level of leptin is high enough, the onset of puberty occurs.
Puberty is driven by a cocktail of hormones:
At the onset of puberty, your daughter’s ovaries produce small pulses of oestrogen every 90 minutes. Over the next month, these pulses of oestrogen get longer and more frequent until there is a continuous tiny stream of oestrogen into the bloodstream.
This triggers the development of the breasts which is the first outward sign that puberty has begun.
Human females are the only mammals that develop breasts well before pregnancy – even decades earlier. In fact, full breast development is only completed at the birth of a baby when the breasts produce colostrum from the milk glands within the breast.
The reason breasts appear before pregnancy in females is probably down to their function as a secondary sexual characteristic – a sign of sexually mature femaleness in a biological sense.
Breast development progresses through three stages:
Usually, breasts develop before pubic hair, and they are mediated by different hormones and glands. Breast development is mediated by oestrogen production in the ovaries.
Underarm hair and pubic hair growth are mediated by androgens from the adrenal glands that sit on top of the kidneys. This adrenal system or adrenarche also promotes the development of sweat production, increased skin sebum production and acne.
In most girls, the early androgen effects overlap with, the earliest oestrogen-driven puberty (which promote breast development and skeletal growth).
However, if your daughter was born ‘small for dates’ because of intrauterine growth restriction she may start developing body hair before her breasts develop. If you or your daughter are concerned about her development speak to her doctor.
Progesterone and oestrogen are required to prepare the uterus for potential implantation by a fertilised egg, and their release is triggered by the hypothalamus (in the brain).
After menstruation has begun the ovaries release one egg each month (the ovaries take turns). This is ovulation.
The hypothalamus sends a signal to the pituitary gland to release follicle stimulating hormone (FSH) and luteinizing hormone LH.
As the egg travels from the ovary, down the fallopian tube, progesterone is released from the site in the ovary where the egg erupted called the corpus luteum. Progesterone prepares the uterus for pregnancy by causing the lining of the uterus to thicken. If a woman is not pregnant, the corpus luteum disappears and the period begins where the lining of the uterus is shed out of the cervix and out of the vagina. A girl will only lose a few teaspoons of blood during a period.
Apart from during pregnancy and breastfeeding girls and women have a period roughly every 28 days. The average woman in a developed country will have 400 periods.
Julianna Deardorff PhD & Louise Greenspan MD (2014) ‘The New Puberty: How to Navigate Early Development in Today’s Girls.’ Rodalle Books
Graber, J. A., Nichols, T. R., & Brooks-Gunn, J. (2010). Putting pubertal timing in developmental context: Implications for prevention. Developmental Psychobiology, 52, 254–262.
Lynne, S. D., Graber, J. A., Nichols, T. R., Brooks-Gunn, J., & Botvin, G. J. (2007). Links between pubertal timing, peer influences, and externalizing behaviors among urban students followed through middle school. Journal of Adolescent Health, 40, 181e7-181e13.