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What is menstrual cycle?

Women menstrual cycle is a part of reproductive system – all reproductive organs are involved in the menstrual cycle (regulated by hormones). Menstrual cycle is determined genetically and hormonally.  Menstrual cycle and menstrual periods start when girls become sexually mature at the time of puberty.

Menstrual cycle is  the monthly cycle in which female body releases an egg, prepares itself for fertilization of the egg (by sperm). At the same time the female body develops a specific environment in the uterus (womb) for implantation of the fertilized egg and for following embryo (baby) development. Actually menstrual cycles prepare female body to become pregnant every month. This is why all modern healthy females need family planning and safe contraception.

Monthly menstrual periods usually happen if woman did not have fertilization (absence of pregnancy). Menstrual bleeding composed of the internal layer of uterus (called “endometrium”) itself, together with a little fresh blood caused by the breaking of very fine blood vessels within the endometrium. The amount of blood lost during normal menstrual period is usually less than 80-100ml.

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 be 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 menstrual 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).

What should be considered as a normal menstrual cycle?

The duration (length) of normal healthy menstrual cycle can vary from a short cycle of only 21 days to a long cycle of 40 days. The length of the menstrual cycle is calculated by counting the first day of menstrual bleeding (Day One) and then counting until the very last day before the next menstrual bleeding (period). Start of next period is again the Day One (for next menstrual cycle). The duration of the normal menstrual period (monthly bleeding) should be in average about 3-5 days.

Menstrual Cycle

Menstrual Cycle

Menstrual cycle phases

Normal menstrual cycle has three phases: follicular phase, ovulation and luteal phase. Female hormones play a crucial role in menstrual cycle.

Follicular Phase

The follicular phase of the menstrual cycle starts from the Day One of menstrual bleeding. During follicular phase ovaries develop a viable follicle capable of undergoing ovulation.  The early events of the follicular phase are initiated by a rise in Follicle-Stimulating Hormone (FSH) levels at the first day of the cycle. The rise in Follicle-Stimulating Hormone levels can be attributed to a decrease in progesterone and estrogen levels at the end of the previous cycle and the subsequent removal of inhibition of FSH by these ovarian hormones. Follicle-Stimulating Hormone triggers and stimulates the development of 15-20 follicles each month. Growing follicles produce estradiol – the level of estradiol is increasing with growing of follicles. By the end of the follicular phase, the increased level of estradiol inhibits the secretion of the Follicle-Stimulating Hormone and FSH levels decrease.

During normal healthy follicular phase of the menstrual cycle, usually the dominant follicle is developed (follicle with mature egg ready for fertilization). The dominant follicle, with its high concentration of FSH receptors, continues to acquire more FSH even as FSH levels decrease. The dominant follicle can continue to synthesize estradiol, which is essential for its complete maturation. The dominant follicle matures and secretes increasing amounts of estrogens. Estrogen levels peak towards the end of the follicular phase of the menstrual cycle. At this critical moment, estrogen exerts positive feedback on Luteinizing Hormone (LH), generating a dramatic pre-ovulation LH surge.

Ovulation

Ovulation is the most important phase of the menstrual cycle. Actually ovulation is needed for pregnancy, ovulation can be considered as fertility indicator and ovulation is a critical period of dramatic hormonal changes in women body.

The LH surge is neccessary for ovulation. The LH surge induces release of proteolytic enzymes, which degrade the cells at the surface of the follicle, and stimulates angiogenesis in the follicular wall and prostaglandin secretion. These effects of LH cause the follicle to swell and rupture. After the rupture, the released mature egg is ready for fertilization – moving from ovary to fallopian tubes (where can meet active spermatozoids).

Luteal Phase

The luteal phase is defined by the luteinization of the components of the follicle which were not ovulated. Luteinization is initiated by the LH surge – the luteinization ends up with so called “corpus luteum” which means absence of fertilization and absence of pregnancy. “Corpus luteum” produces hormone progesterone. Progesterone secretion by the “corpus luteum” reach the maximum (peak) between 5-7 days post-ovulation.

Duration of the luteal phase is always 14 days. After 14 days “corpus luteum” undergoes atresia and begins evolving into the so called “corpus albicans”. With the decline of both estrogens and progesterone levels, an important negative feedback control on FSH is removed and FSH levels rise once again to initiate the next menstrual cycle.


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