Читать книгу The Ultimate PCOS Handbook - Theresa Cheung - Страница 28

The first ovulation doesn’t occur until around six to nine months after a girl’s first period.

The mechanisms for initiating menstruation are present in a child’s brain and pituitary gland from birth, but something keeps them turned off until puberty. Many scientists believe this is weight, with a body-fat ratio of around 25 per cent being the trigger. It’s thought that the hormone leptin, produced by fat cells, must achieve a certain level in the blood in order for menstruation to occur, and that this level must be sustained for the menstrual cycle to be regular.

As body weight during puberty increases it’s thought that the hypothalamus triggers the release of gonadotrophin-releasing hormone (GnRH). It’s thought that the body fat ratio triggers the onset of menstruation, some time between the ages of 11 and 18.

GnRH then stimulates the pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). As we’ve seen in Chapter 1, in sexually mature females FSH and LH act on the follicle to stimulate it to release oestrogens and trigger the maturing of the egg and its release (ovulation) in the middle of the cycle. Later, LH stimulates the empty follicle to develop into the corpus luteum, which secretes progesterone during the latter part of the menstrual cycle to prepare the body for a possible pregnancy.

Somehow the brain senses the body mass or fat mass, and only lets puberty start when a certain weight is reached, typically around 7 stone (100 lb), although this can vary considerably. This would explain why petite girls often get their periods later than heavier or taller ones. It would also explain why some women stop getting periods when they lose too much weight (think of gymnasts, or women with anorexia or even athletes in hard training). Although this theory has its problems, as some very thin women do menstruate, it does make sense that pregnancy shouldn’t occur until a body has enough fat stores to see it through successfully.

Research⁴ indicates that a healthy diet with a balance of fats, proteins and carbohydrates is important for the onset and continuation of menstruation, whether a woman has PCOS or not. In the 1830s women typically began menstruation at the age of 17, while today the average age is 13. This change is thought to be linked to improved nutrition, but some argue that it’s due to rising obesity levels and exposure to hormones in the environment and to food additives.

Once menstruation has started, a continual release of the hormone GnRH is essential to keep periods regular. Stress and sudden body weight changes can upset the release of this hormone and cause irregular or absent periods. Again, perhaps this is the body’s way of ensuring that pregnancy only occurs when a woman’s mind and body are ready to support it.