In females, puberty usually begins between the ages of eight and 13. As a girl changes into a woman, her hips widen and her breasts swell, as fat accumulates and milk duets form inside them. It is 18 months to two years after the start of these signs of puberty that a girl reaches the menarche, when her pituitary gland starts sending out hormones and the menstrual cycle begins.
Periods are often irregular for the first year or two after puberty, because girls don't always start releasing mature eggs immediately. It can take a couple of years before regular ovulation and a fixed menstrual pattern are established.
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The menstrual cycle
The word menstruation comes from the Latin mensis, or month, and the menstrual cycle (the system of maturing and releasing eggs) is generally considered to be a 28-day process. In fact, each woman has her own individual rhythm and may have a cycle of anything between 25 and 35 days. Some women have even longer or shorter cycles, and many have irregular periods, which can indicate that they are not ovulating normally. The menstrual cycle is regulated by messages sent from the brain. They tell your body when to create the hormones to trigger each stage of the cycle.
The period
The monthly cycle begins when your period starts. The days of the cycle are numbered from the first day of bleeding, so this is known as day one. Your body prepares a soft lining, or endometrium, for your womb every month, just in case you get pregnant. If there is no fertilized egg attached to the womb lining, the levels of progesterone in your body start to fall, and the blood vessels in the endometrium contract. This causes the lining to break up, and it is shed through the neck of the womb, or cervix, into the vagina.
A period may last up to a week, although between three and five days is more usual. It is common to experience some pain or cramping during your period, caused by contractions in the muscles of the womb. One in five women suffer with very heavy bleeding, or menorrhagia, which is defined as a period that lasts more than seven days or during which more than 80 millilitres (2 1/2 fluid ounces) of blood is lost. Most of us have absolutely no idea how many millilitres of blood we lose, or whether our periods are light or heavy compared to other women's. There are wide variations in what is described as 'average' blood loss during a period, but it seems to be anything between a few spoonfuls and half a cup.
During your period, the hypothalamus gland in the brain starts to release a hormone (the gonadotrophin-releasing hormone or GnRH, for anyone who likes to know the names of these things). This signals the pituitary gland to send out follicle-stimulating hormone, or FSH, Which makes the eggs in the ovaries start to grow.
Leading up to ovulation - follicular phase
Initially, as many as 20 eggs may respond to the FSH. Each egg is contained in a little fluid-filled sac, called a follicle. As the follicles start to grow, they send out another hormone: oestrogen. One follicle becomes dominant by about day seven of the cycle, growing faster than the others and releasing more oestrogen. Once the oestrogen reaches a certain level, this alerts the pituitary gland to stop producing so much FSH. The other follicles will then shrink and the eggs inside them die, leaving just the dominant follicle to continue growing. Meanwhile, the lining of the womb, or endometrium, is thickening so that it will be ready in case there is a fertilised embryo to implant.
Ovulation
Once oestrogen levels have reached their highest point, it is time for an egg to be released. It normally takes 14 days from the start of the period to ovulation, but it can take longer for the egg to mature. When the egg is ready, the pituitary gland sends out huge amounts of luteinising hormone, or LH, in a surge that will last 24 hours. The follicle ruptures and the egg is released. Some women know when they ovulate, as they have what is known as Mittelschmerz, or mid-cycle pain. It is usually felt in the lower abdomen, often just inside the hip bone.
The newly released egg is swept up by the fimbria and gathered into the funnel at the end of the fallopian tube, and then begins to travel towards the womb. An egg can survive for only 24 hours once it has been released, so it needs to be fertilized quickly. To find out more, you can check out Am I Infertile.
