Duration
A normal menstrual cycle can be anything from 24 to 36 days, not necessarily the 28-day cycle quoted in text books. Cycles can sometimes be even longer or shorter than these common boundaries, and even the most regular of women may sometimes experience unusually long cycles, usually when stress or a similar factor has delayed ovulation.
Stages of the cycle
It's important to know that your cycle is split into two main parts, the first is the follicular, or oestrogenic, phase from your period up to the time of ovulation. The second part is the luteal, or progestational, phase, from ovulation until the first day of your next period. Much of the mystery surrounding late periods and suspected pregnancies can be removed if you understand that while the second, or luteal phase is around the same length each cycle, usually 12-16 days, the length of the first phase of the cycle can vary.
The osetrogenic phase
The menstrual cycle involves the interplay of several different reproductive hormones which in the first phase of the cycle act together to ensure the maturing and release of an egg from one of your two ovaries. Each cycle the Gonadotrophin-releasing hormone produced in your brain prompts your pituitary gland to release Follicle Stimulating Hormone, or FSH, into the bloodstream. The FSH stimulates the ovaries to start maturing eggs, around 15-20 eggs in each ovary, in individual follicles that produce oestrogen. As well as encouraging the eggs within the follicles to mature, this oestrogen stimulates the lining of the womb, or endometrium, to thicken in preparation for a possible conception.
The time it takes the eggs to mature is usually about two weeks, though it can be as little as 8 days, as long as a month, or sometimes even longer. As the oestrogen level reaches a threshold it causes the pituitary gland to release a flood of luteinising hormone (LH), commonly known as the LH surge, which in turn triggers the release of an egg from the most mature follicle. The egg bursts out of the ovary and into the pelvic cavity: This is ovulation.
Ocasionally two eggs are released within hours of each other and can result in fraternal twins if both are fertilised.
The Luteal phase
Once the egg is released into the pelvic cavity it is quickly 'caught' by the finger-like fimbria at the ends of your fallopian tube and drawn into the tube itself. The egg is then propelled along the fallopian tube by the vibrations of tiny, hair-like cilia on its walls.
Meanwhile the follicle that released the egg collapses, becoming a 'corpus luteum', or yellow body and starts releasing progesterone, which within 24 hours will prevent any further eggs being released during that cycle. The progesterone also encourages the thickening of the endometrium in preparation for conception and is responsible for physical changes that you look out for when charting in: waking temperature, cervical mucus and cervical position. The corpus luteum lives for between 12 and sixteen days, determining the length of the luteal phase.
If the egg is not fertilised
The egg will live at most for 24 hours if it isn't fertilised and will then be either be reabsorbed by the body or will be expelled along with menstruation. If the egg isn't fertilised the corpus luteum will die at the end of its normal lifespan and will no longer release progesterone. The levels of progesterone and oestrogen then fall, your womb contracts (which is the cause of period cramps) and the lining of the womb starts to disintegrate, starting your menstrual flow.
And then the whole process can start anew!
If the egg is fertilised
If conception takes place it will usually be within a few hours of ovulation and in the upper part of the fallopian tube. Sperm swim up through the cervix and womb and into the fallopian tubes - of the millions that are released in ejaculation only one will successfully penetrate the walls of the egg, or ovum, and fuse with it to form the zygote.
The zygote immediately divides into two cells, then four, then eight and so on. This continually dividing and growing cluster of cells, now called the morula, is swept down inside the fallopian tubes towards the uterus. By the time the morula reaches your uterus, about four days after conception, it has grown to around 100 cells.
The bundle of cells then burrows its way into the thickened lining of your uterus and starts to produce the pregnancy hormone HCG (Human chorionic gonadotrophin), which alerts the corpus luteum to prolong it's usual lifespan and continuing releasing progesterone so that the lining of the womb will not disintegrate and can continue to provide a nourishing environment for the embryo.
An anovulatory cycle
An anovulatory cycle is when your body is unable to reach the oestrogen threshold necessary to trigger ovulation, so ovulation doesn't occur.
You can still have what appears to be a period when you're not ovulating, either because a fall in oestrogen levels triggers oestrogen withdrawl bleeding, or when the lining of the uterus builds to an unsustainable level and then begins to disintegrate. If you're not charting your temperature then you might not realise that you haven't actually ovulated.
It's more obvious that you're not ovulating when your periods stop altogether for a time, resulting on one, very long, cycle. Common causes of prolonged anovulation include illness, certain medical conditions, travel, stress, heavy exercise (usually athletes) or low body weight.
There are also certain times in your life when you might experience anovulation. The most obvious are pregnancy and the menopause, when ovulation stops altogether. Adolescent girls also often have anovulatory and irregular periods until their cycles settle down, and breastfeeding mothers are unlikely to ovulate, which is nature's way of helping to space pregnancies healthily.
Find out more about anovulation
