Medically Reviewed on April 24, 2013 by George Krucik, MD, MBA
Written and medically reviewed by the Healthline Editorial Team
Co-developed by:

In Depth: Sagittal Cross-section

The end result of the human reproductive system is a baby, and women bear the brunt of the work in the process.

Still, the process of conceiving a baby is a delicate one depending on various fluctuations of hormones, sex cells, and the response of organs. Unlike men, who generate sperm continuously, an egg only develops in a female about once a month.

During this cycle, women experience hormonal changes, including a fluctuation of the following hormones:

  • Estrogen: The hormone that gives a woman her womanly qualities also helps her reproductive cycle by aiding in certain functions of the reproductive system, especially thickening the inner membrane of the uterus to prepare for a fertilized egg and other functions during menstruation.
  • Progesterone: This steroid hormone aids in readying the uterus for a fertilized egg and prevents fetus rejection. Manufactured forms of progesterone are used in several birth control devices.
  • Estradiol: A type of estrogen, this is the predominant sex hormone in females. It also prepares the lining of the uterus for an embryo to attach to the uterine wall.

The ovaries contain about half a million cells capable of becoming eggs. These cells mature in stages. In the middle of each menstrual cycle, an ovarian follicle becomes stimulated by follicle-stimulating hormones (FSH) produced by the pituitary gland and releases an egg.

The egg and its surrounding cells move down into the fallopian tube to await fertilization. The short period of time an egg is available and during which a woman may become pregnant is known as ovulation.

Once ovulation begins, estrogen levels decrease and progesterone levels rise during the next seven days. If the cycle concludes without fertilization, all hormone levels drop significantly as the woman begins menstruation, the regular shedding of the uterine lining and unused egg through the vagina.

Many women experience cramps during this period as a result of the uterine muscle contracting to expel the uterine lining.

Other common symptoms — often referred to as premenstrual syndrome, or PMS — include bloating, fatigue, headache, breast tenderness, aggression, depression, mood swings, and inability to concentrate. These typically occur between ovulation and the start of menstrual bleeding.

Women experience changes or irregular patterns in their menstrual cycle for several reasons including:

  • Using birth control
  • Pregnancy
  • Extreme weight loss
  • Stress
  • Polycystic ovarian syndrome

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