The menstrual cycle is the genetically and hormonally determined cycle of development discovered in women reproductive system. Menstrual cycle is very specific and individual, and can range 24 days to 37 days. Duration is not so important but the regularity is very important. At the beginning cycles could be not regular – it is a period of development. From age 18-20 usually cycles are already fully developed and periods should very regular. Just remember – normal cycle is what is normal for your body. The length of your cycle may be affected by many things, including illness, stress, travel, fertility medication, and some other factors.
If your cycle is 28-day cycle, the first day of bleeding should be considered as a first day of the cycle. It is important to remember because in many cases (disturbances, pregnancy, contraception use, etc.) the first day of the cycle is usually the start of calculations (for you and for medical staff).
Every month female ovaries begin to ripen several follicles, which is why days 1-14 are called the follicular phase. Each of the many follicles in the two ovaries is a pocket of tissue filled mostly with estrogens. The number of active follicles changes with each cycle, but typically only one follicle per month in a single ovary becomes dominant over the others and produces a viable egg. As the follicles ripen, estrogen levels rise. Just before mid-cycle, the hypothalamus and pituitary gland release LH (luteinizing hormone) and FSH (follicle stimulating hormone) spikes to trigger the dominant follicle to ovulate. Estrogen levels continue to rise. Days 14-28 are termed the luteal phase, when estrogen levels begin to fall and progesterone levels rise. Around day 14 (but it can vary) ovulation occurs: the dominant egg is released from the follicle and is drawn into the fallopian tube on its way to the uterus. This triggers a host of hormonal secretions – including estrogen – that thicken the uterine lining to support a pregnancy.
Important to remember – women are fertile for about 24-48 hours around the time of ovulation. Progesterone levels remain high unless the egg goes unfertilized, in which case the egg is re-absorbed and progesterone levels fall. In this event, progesterone levels continue to fall until day 28, when progesterone reaches its lowest level, menstruation occurs, and the cycle repeats. This turning point almost always occurs 14 days after ovulation.