Infertility Frequently Asked Questions
Understanding Female Fertility
For a woman, fertility refers to the ability to become pregnant and have a baby. A woman's reproductive years begin when she has her first menstrual cycle (around age 12), and usually ends around age 45 when her ovarian reserve (store of eggs) is depleted, although it's theoretically possible for a woman to become pregnant until her periods end with menopause (around 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. As she enters her reproductive years (menarche), she starts having menstrual cycles. Many teenagers have irregular periods, but by the time a woman enters her twenties, her natural monthly cycle should have established itself and become fairly regular. During each cycle, the ovary releases one egg (or occasionally more than one), which may join with a man's sperm and begin a pregnancy.
For each follicle that eventually ovulates, close to 1,000 will have a limited but unsuccessful growth. The number of eggs that are lost per ovulatory cycle probably varies throughout a woman's life, but is presumed to be accelerated in the 10–15 years that precede menopause. At this time, not only are the numbers rapidly dwindling, but there is also a decline in the quality of the follicles. Hormonal production is not as predictable and robust and the eggs often contain subtle mutations in their genetic make-up.
The decline in the quantity and the quality of eggs during this time explains the diminishing fertility that is seen in women from age 35 and onward. Even when an older woman does conceive, her chances of having a miscarriage increases greatly due to chromosomal damage to her eggs. Moreover, an older woman is much more likely to have developed endometriosis, fibroids, or other abnormalities of the reproductive system that can cause problems with fertility.
During a menstrual cycle, 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).
In the early part of a cycle, FSH triggers the growth of 10 to 20 follicles in the ovary, although usually only one of them will mature fully. As the follicles (and the eggs inside them) develop, the ovary increases its production of the hormone estrogen. Estrogen signals the uterus 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 (usually) one follicle and its egg reach maturity, 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 been maturing 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 or insemination within the past few days, the 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 will continue to travel down the tube and into the uterus. During this time, it will begin 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), in 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, meaning "yellow body" because it actually turns yellow) 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 to 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.