Understanding Your Menstrual Cycle 
You probably have a sense of how often you get your period. Typically, women have cycle lengths that vary in length between 23 and 35 days long. Any variation in cycle length that does occur is usually during the part of the cycle before you ovulate. Most women then have 12–16 days between the time of their ovulation and the start of their next period.
In technical terms, the first day of your menstrual cycle is the first day of your period. Your period usually then lasts anywhere from 3 to 7 days. You will probably find that should you have menstrual discomfort, any pain you may be experiencing tends to be at its worst on the first day of your period. This is due to the timing of the release of hormones in your body that stimulate your womb to shed the blood-thickened lining that was built up in the previous cycle. The shedding of this blood is what you think of as your period.
At the same time, your fertility cycle starts over again as your body sends a signal to your brain to start producing follicle-stimulating hormone (FSH), increasing the amount of estrogen in your body and causing one of your ovary's eggs to start ripening within its follicle. The follicle gets bigger and, meanwhile, the increasing amount of estrogen in your body is ensuring that the lining of your womb begins the process of re-thickening with nutrients and blood. If you do get pregnant, the fertilized egg will have all the nutrients and support it needs to grow. High estrogen levels are also associated with the appearance of "fertile cervical mucus", which you may notice as a thin and slippery discharge that may be cloudy white. "Sperm-friendly", this mucus allows sperm to swim more easily towards the waiting egg, surviving in its journey for several days.
The level of estrogen in your body continues to increase, and will eventually trip a rapid rise in Luteinising Hormone (In fertility lingo: an "LH 'surge"'). This LH surge gives the ripening egg the final push it needs to fully ripen and to be released from its follicle. This is ovulation. Although many women think that they ovulate on day 14 of their fertility cycle, this is not always the case. Your day of ovulation will vary depending on your cycle length. Some women feel a twinge of pain when they ovulate but otherwise, there will not be any other sign that you are actually ovulating.
Once your egg has been released, it moves along one of your Fallopian tubes towards your womb. Without fertilization, the egg can survive for up to 24 hours. At the same time, sperm can live for several days. There are two days, therefore, the day before ovulation and the day of ovulation are when you are at your most fertile and most likely to get pregnant.
As soon as the follicle releases your egg, the follicle starts producing another hormone called progesterone. Progesterone now works to further build up the lining of your womb to prepare for a fertilized egg. Meanwhile, the empty follicle starts to shrink, producing both progesterone and estrogen. Symptoms of pre-menstrual syndrome (PMS) such as breast tenderness, bloating, lethargy, depression and irritability start during this phase of the cycle.
As the empty follicle continues to shrink, if the egg it has released is not fertilized, your levels of estrogen and progesterone decrease markedly because they are no longer needed. Without the high levels of hormones to help maintain it, the thick womb lining that has been built up starts to break down and your body will shed the unneeded lining. This is the start of your period and the beginning of your next fertility cycle.
If, on the other hand, your egg is fertilized, the now empty follicle is kept from shrinking by the pregnancy hormone (human Chorionic Gonadotrophin), continuing to produce estrogen and progesterone for about eight weeks until the placenta is mature enough to provide the embryo with all the nutrients it needs.
See an animation of how the menstrual cycle
works
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