The menstrual cycle is a result of the complex interaction of several hormones. Many hormones are involved, but the four major ones are follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen and progesterone. The interaction of these hormones generally affects two areas of the reproductive system the most: the uterus and the egg follicles in the ovaries.

The menstrual cycle can be divided into two major phases:

• The follicular phase. This phase includes the first half of the menstrual cycle, beginning with the menstrual period and ending with ovulation. In a 28-day cycle, this includes the first 14 days. The first day of the menstrual period is counted as day one of the cycle. The uterine lining (endometrium) is shed during the menstrual period for about the first five days of the follicular phase.  
  
  Two hormones are dominant during this follicular or proliferative phase:
  
  
• Follicle stimulating hormone (FSH). Produced in the pituitary gland in the brain. FSH stimulates about 15 to 20 egg follicles to begin developing in one ovary. FSH stimulates the follicle development and also stimulates the follicles to produce estrogen.
  
  
• Estrogen. Produced by the growing follicles. As estrogen reaches a certain level, it causes the endometrial lining (which was just shed as menstrual blood) to begin to grow again. As the estrogen level rises, it eventually triggers a stop in FSH production, which prevents more follicles from developing.
  

During the follicular phase, one developing egg follicle becomes larger than the others (dominant follicle). At about day 14, the estrogen from this follicle triggers a surge and release of LH from the pituitary gland, initiating ovulation. The egg follicle bursts and releases the mature egg into the fallopian tube.

• The luteal phase. Includes the remaining 14 days of the menstrual cycle (when a woman has a 28-day cycle), beginning with ovulation and ending with the start of the next menstrual period. Two other hormones are dominant during the luteal phase:
  

• Luteinizing hormone. Produced by the pituitary gland. Small amounts of LH are produced early in the follicular phase, but the later surge of LH causes ovulation. After ovulation, LH acts on the empty egg follicle, now called the corpus luteum. The corpus luteum secretes small amounts of estrogen and greater amounts of progesterone.
  
  
• Progesterone. Produced by the corpus luteum. Progesterone promotes the growth and development of the endometrium to receive a fertilized egg. The progesterone level peaks at about day 21 of a 28-day cycle. If there is no fertilized egg, progesterone levels decrease, causing the uterine lining to begin to break down after several days. This subsequently results in menstruation.  
  

The production of estrogen also decreases after ovulation, as the follicles degenerate. With less estrogen and progesterone to promote endometrial development, the uterine lining breaks down enough to be shed by the end of the luteal phase at day 28. This begins the next menstrual period and returns the menstrual cycle to day one.

The days in the middle of the menstrual cycle, near ovulation, are the most likely time for a woman to get pregnant during sexual intercourse. However, in some women ovulation occurs at different times in the cycle. Women can become pregnant while having their periods. The variability in menstrual cycle length in women makes the precise identification of where one woman is in her particular cycle difficult.

If the egg is fertilized in the fallopian tube, it typically continues to the uterus. The enriched endometrium provides a place for the fertilized egg to implant. It does not shed and leave the body and this missed period may be the first sign of pregnancy.

However, some women may experience a small amount of spotting or bleeding very early in the pregnancy, about 10 to 14 days after fertilization, which may be mistaken for menstruation. This is known as implantation bleeding and occurs when the fertilized egg first attaches to the lining of the uterus. This type of bleeding is usually a bit earlier and lighter in color than a usual period and does not last long.