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Normal Female Fertility
In women, fertility means the ability to become pregnant and have a baby. A woman's reproductive years begin when she starts her menstrual cycles during puberty (about age 13). The ability to have a child usually ends around age 45, though it's potentially possible for a woman to get pregnant until her periods end with menopause (about age 51).
When a baby girl is born, she already has in her body about 400,000 oocytes (immature eggs). These are stored in her ovaries in tiny fluid-filled sacs called follicles. Once she enters her reproductive years, she starts having monthly menstrual cycles. During each cycle, the ovary releases one egg (or, less commonly, more than one), which may go on to join with a man's sperm cell and begin a pregnancy.
The development and release of the egg depend on a delicate balance of hormones — chemicals that signal the body's organs to do particular jobs. Some of these hormones are produced in the ovaries. Others come from two glands in the brain, the hypothalamus and the pituitary.
At the beginning of each menstrual cycle, the hypothalamus releases gonadotropin releasing hormone (GnRH) into the bloodstream. This hormone prompts the pituitary to release the proper amounts of two more hormones — follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
First, FSH causes some of the follicles in the ovary to begin to grow. As the follicles (and the eggs inside them) develop, the ovary increases its production of the hormone estrogen. Estrogen signals the uterus (womb) to build up a thick, blood-rich lining so it will be ready to sustain a pregnancy.
Then, around the middle of the cycle, when at least one follicle and its egg are mature, the pituitary releases a surge of LH. In response, the follicle breaks open, releasing the egg. This event is called ovulation. Normally, only one egg is released. The other eggs and follicles that had started growing but were not released are absorbed by the body.
Once the egg leaves the ovary, it enters the flower-like opening of the nearby fallopian tube and travels down the tube toward the uterus. If the woman has had sexual intercourse within the past few days, her partner's sperm may be in the fallopian tube. If so, then one of these sperm may enter the egg, fertilizing it. The fertilized egg is the first cell of a new human being.
The fertilized egg continues to travel down the tube and into the uterus. During this time, it begins to divide, over and over, forming a small clump of cells. Within a few days, this clump of cells attaches to the thickened uterine lining (endometrium), a step called implantation. The growing ball of cells is now called an embryo.
The high LH level also stimulates the newly ruptured follicle on the ovary (now called the corpus luteum) to secrete yet another hormone, progesterone. The presence of progesterone keeps the endometrium thick and stable, so it can support the growing embryo. Meanwhile, the implanted embryo begins to develop a placenta, the organ that will connect the developing baby with the mother's body. The new placental tissue produces a hormone called human chorionic gonadotropin (hCG). This "pregnancy hormone" tells the corpus luteum to continue making progesterone, which keeps the endometrium in place. After a few weeks, the growing placenta takes over the job of producing progesterone, and the corpus luteum slowly withers away.
If the egg is not fertilized as it passes through the fallopian tube, there is no embryo to implant in the uterus and produce hCG. Toward the end of the cycle, the corpus luteum stops producing progesterone. The drop in progesterone signals the endometrium that it's not needed to support a pregnancy this month. The uterus sheds the blood-rich endometrial tissue in a process called menstruation, causing a menstrual period. Then, the cycle repeats itself.
The Eye and How It Works
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