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The Menstrual Cycle

While you may hate them, the raging hormones of your menstrual cycle are what prepare your ovaries for ovulation and your uterus for a baby. Your menstrual cycle is controlled by hormones and can be divided into three parts: Menstruation (Period), Post-Menstruation, and Luteal Phase. The graph above shows a typical 28-day menstrual cycle.

Menstruation (Period)

The typical menstrual period lasts 4 days (days 1-4 of the menstrual cycle). During menstruation, the lining of your uterus (endometrium lining) sheds and small pulses of GnRH from your hypothalamus causes your pituitary gland to secret the hormone FSH (follicle stimulating hormone) which causes immature follicles, each of which contains an unfertalized egg, to begin growing in your ovaries.

Post-Menstruation

The Post-Menstruation Phase begins when your menstrual period ends and ends at ovulation. In a typical 28-day cycle, post-menstruation lasts 10 days (days 5-14). However, this part of your cycle may vary in length due to stress, illness, diet, or increased physical activity. When it varies in length, so too will your cycle. During post-menstruation, FSH continues to cause the stimulated follicles to grow in your ovaries. The stimulated follicles produce estrogen in increasing amounts, which causes the lining of your uterus to thicken. Typically, one follicle becomes dominant, producing more estrogen than the others, killing the other follicles. When it reaches maturity, estrogen levels rise which tells your brain that it is time for ovulation. When the egg is ready, your hypothalamus secretes massive amounts of GnRH causing your pituitary gland to release LH (Luteinizing hormone) over a 24-48 hour period (known as the "LH surge"), which triggers ovulation.

Ovulation

In a typical 28-day cycle, ovulation occurs between day 14 and 15. During ovulation, the "LH surge" causes each follicle that reached maturity during post-menstruation to expel its egg. After the egg is expelled, it is capable of being fertilized for 12 to 48 hours before it begins to disintegrate.

Luteal Phase

The Luteal Phase begins at ovulation and is pretty consitent in length, even when your menstrual cycle is not. In a typical 28-day cycle, it lasts 14 days (day 15-28). At the beginning of this phase, the "LH surge" causes the remainder of the follicle that released its egg to be converted into a corpus luteum. This corpus luteum produces the hormone progesterone, which prepares the lining of your uterus (endometrium lining) for a possible implantation of the egg. Your egg travels down your fallopian tube and implants in your uterus (day 15-20). If it was not fertilized, the corpus luteum will completely disintegrate, progesterone production will cease and menstruation will occur again. If the egg was fertilized, the embryo will produce the hormone hCG (Human chorionic gonadotropin) which will prevent the disintegration of the corpus luteum so that it continues to produce progesterone, which is critical for a pregnancy in humans.

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