by Alan Penzias, MD
Infertility is a common but frequently treatable condition. It affects one couple out of six and becomes more common with increasing age. Traditionally, infertility has been defined as one year of unprotected intercourse without conception. However, there are many exceptions to this rule. In order to maximize your chances of becoming pregnant you need to know when it is time to seek help. To be your own best advocate, it is helpful to have a basic knowledge of the comprehensive infertility evaluation before even going to the doctor.
The truth about eggs
Every woman is born with all of the eggs that she will ever have. Once the eggs are gone, they can't be replaced. At birth, a girl has approximately one million eggs in her ovaries, but by the time she has her first period only about 400,000 remain. Throughout her reproductive lifetime, a woman will ovulate (release an egg from her ovary) approximately 400 times. This means that roughly 1000 eggs are stimulated to develop each month but only one survives and is released from the ovaries. The other 999 eggs simply wither away.
The hormones that control your ovaries
In order for an egg to be released each month, a woman's body produces and releases the hormones luteinizing hormone (LH) and follicles stimulating hormone (FSH) in a very precise and well-controlled way. Just before the egg is released, the LH rises to a level of three or four times its baseline value. This temporary rise of LH causes the ovary to produce the hormone progesterone. At the same time, the egg, which has grown inside a small pouch of cells on the ovary called a follicle, completes its maturation and is released.
Production of progesterone in the ovary is an important event because progesterone causes a rise in a woman's body temperature. The rise in temperature is small (often less than one degree farenheit) but may be detected using a sensitive thermometer that is available in most pharmacies. This is important because it means that most women have a tool available to help them determine when an egg has been released. It may be helpful to use an online chart such as Pregnancy.org's BBT.
Where do the eggs go?
Once the egg has been released by the ovary it is picked up by the fimbria or "fingers" of the fallopian tube. Although the right fallopian tube commonly picks up an egg from the right ovary and the left fallopian tube commonly picks up an egg from the left ovary, either tube can pick up an egg from either ovary.
The story of fertilization
In order for a woman to become pregnant, the egg must meet sperm inside the fallopian tube. Sperm begin their journey to the fallopian tube in the vagina following ejaculation from sexual intercourse. The sperm are carried into the vagina in a complex liquid called semen. The semen contains proteins and buffers that nourish and protect the sperm.
However, the proteins and buffers present in the semen are not meant to enter a woman's uterus (womb). The cervix (which is the entrance to the uterus) is filled with mucous which acts as a filter. This mucous allows only the sperm to enter and keeps the proteins and buffers of the semen in the vagina. From the cervix, the sperm swim through the uterus and into the fallopian tubes.
When sperm and egg meet in the fallopian tube fertilization occurs. The egg is covered by a protective shell called the zona pellucida. Many thousands of sperm attach to the outside of the zona pellucida but only one ultimately enters. When that single sperm gains access to the egg, the zona pellucida hardens preventing additional sperm from entering. After the sperm and the egg are joined, the new cell is called an embryo.
After fertilization, the embryo begins to divide. It goes from one cell to two cells, then from two cells to four cells, and so on as cell division continues. The embryo also begins to migrate down the fallopian to the uterus. The hormonal changes that occurred prior to the release of the egg from the ovary also affect the lining of the uterus (endometrium). These hormonal changes prepare the uterine lining for the arrival of the embryo.