Fertility |
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Conception: How It Works
In order for a pregnancy to occur, four steps must be accomplished:
Sperm Transport
The transport of sperm is dependent upon several factors:
- The sperm must be capable of propelling themselves through the environment of the female vagina and cervix.
- This environment, which is under cyclic hormonal control, must be favorable to admit the sperm without destroying them.
- The sperm must possess the capability of converting to a form that can penetrate the cell membrane of the egg (capacitation).
Following ejaculation, the semen forms a gel which provides protection for the sperm from the vagina. The gel is liquefied within 20 to 30 minutes by enzymes from the prostate gland. This liquefaction is important to free the sperm so transportation may occur. The seminal plasma is left in the vagina. The protected sperm with the greatest motility travel through the layers of cervical mucus that guard the entrance to the uterus. During ovulation, this barrier becomes thinner and changes its acidity creating a friendlier environment for the sperm. Once the sperm have entered the uterus, contractions propel the sperm upward into the fallopian tubes. The first sperm enter the tubes minutes after ejaculation.
Egg Transport
Egg transport begins at ovulation and ends once the egg reaches the uterus. Following ovulation, the fimbriated, or finger-like, end of the fallopian tube sweeps over the ovary. Adhesive sites on the cilia, which are located on the surface of the fimbriae, are responsible for egg pickup and movement into the tube. The cilia within the tube and muscular contractions resulting from the movement of the egg, create a forward motion. Transport through the tube requires about 30 hours.
Once the egg arrives at a specific portion of the tube called the ampullar-isthmic junction, it rests for another 30 hours. Then it begins rapid descent to the uterus. This period of rest in the tube appears to be necessary for full development of the fertilized egg and preparation of the uterus for receipt of the egg. Defects in the process may increase the risk of tubal, called ectopic, pregnancy.
Fertilization
Following ovulation, the egg is capable of fertilization for approximately 12 to 24 hours. Fertilization occurs in the ampulla, or the widest segment of the fallopian tube. Contact between the egg and sperm is random. A membrane surrounding the egg, called the zona pellucida, has two major functions in fertilization. First, the zona pellucida contains sperm receptors which are species specific, or specific for human sperm. Second, once the membrane has been penetrated by the sperm, it becomes impermeable to penetration by other sperm. Following penetration, a series of events set the stage for the first cell division.
Implantation
Once the embryo reaches the blastocyst stage, which is approximately five to six days after fertilization, it hatches out of its zona pellucida and begins the process of implantation. In nature, 50 percent of all fertilized eggs are lost before a woman's missed menses. So, too, in the in vitro fertilization (IVF) process an embryo may begin to develop but not make it to the blastocyst stage -- the first stage where those cells destined to become the fetus separate from those that will become the placenta. The blastocyst may implant but not grow or the blastocyst may grow and still cease development before the two week time at which a pregnancy can be detected. The receptivity of the uterus and the health of the embryo are important for the implantation process.
More Information:
Reviewed by health care specialists at UCSF Medical Center.
Last updated May 8, 2007
This information is for educational purposes only and is not intended to replace the advice of your doctor or health care provider. We encourage you to discuss with your doctor any questions or concerns you may have.
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