The most common test of pregnancy involves the detection of a hormone known as human chorionic gonadotropin (hCG) in a sample of blood or urine.
To determine whether or not a woman is pregnant.
Shortly after a woman's egg is fertilized by her male partner's sperm and is implanted in the lining or the womb (uterus), a placenta begins to form. This organ will help nourish the developing new life. The placenta produces hCG, whose presence, along with other hormones, helps maintain the early stages of pregnancy. Because hCG is produced only by placental tissue and the hormone can be found in the blood or urine of a pregnant woman, it has become a convenient chemical test of pregnancy.
After implantation, the level of detectable hCG rises very rapidly, approximately doubling in quantity every two days until a peak is reached between the sixth and eighth week. Over the next ten or more weeks, the quantity of hCG slowly decreases. After this point, a much lower level is sustained for the duration of the pregnancy. Detectable levels of this hormone may even persist for a month or two after delivery.
Blood tests for hCG are the most sensitive and can detect a pregnancy earlier than urine tests. Blood tests for hCG can also distinguish normal pregnancies from impending miscarriages or pregnancies that occur outside of the uterus (ectopic pregnancies).
If a woman misses her menstrual period and wants to know if she may be pregnant, she can purchase one of many home pregnancy test kits that are currently available. Although each of these products may look slightly different and provide a different set of directions for use, each one detects the presence of hCG. This indicator contains chemical components called antibodies that are sensitive to a certain quantity of this hormone.
Although home pregnancy tests may be advertised as having an accuracy of 97% or better, studies indicate that, in practice, pregnancy tests performed in the home may incorrectly indicate that a woman is not pregnant (a false positive result) between 25-50% of the time. Studies also indicate that the false negative results are usually the result failing to follow the package directions or testing too soon after a missed menstrual period. Waiting a few days after the missed period was expected can increase the accuracy of the test. Blood and urine tests performed by a laboratory are from 97-100% accurate in detecting pregnancy.
Generally, no preparation is required for a pregnancy test given in a doctor's office.
Home pregnancy test kits can be divided into two basic types. One type involves the use of a wand-like device that a woman must place into her urine stream for a brief period of time. The other type of kit involves the use of a cup, a dropper, and a wand or stick with a small well. The cup is used to collect the urine, and the dropper is used to transfer a specific number of drops into the well. Results are displayed by a color change. It's important to follow the package directions very carefully (the techniques vary from brand to brand) and to read the results in the time specified.
No special care is required after a urine test for hCG. Women who feel faint or who continue to bleed after a blood test should be observed until the condition goes away.
Tests for hCG levels pose no direct risk to a woman's health. The main risk with a home pregnancy test is a false negative result, which may be lessened by following the manufacturer's instructions carefully and waiting at least several days after the expected menstrual period to test. A false negative result can cause a delay in seeking for prenatal care, which can pose a risk to both the woman and the baby.
In most cases, a positive result is an indication of pregnancy. However, false positive results may also occur. If a pregnancy test is performed within a month or two of a recent birth or miscarriage, it is possible to test positive for pregnancy since hCG may still be detected in a woman's urine. Sometimes positive pregnancy tests provide clues of an early miscarriage that might have otherwise gone unrecognized because it occurred before or just after a missed period. An ectopic pregnancy (one in which an embryo implants outside the uterus), certain types of masses (such as an ovarian tumor or a hydatidiform mole), and the use of some fertility drugs that contain hCG are among other possibilities behind false positive results.
A woman should notify her physician immediately if her home pregnancy test is positive. Pregnancy can then be confirmed with hCG urine or blood tests taken in the doctor's office and evaluated by laboratory personnel. If performed accurately, home pregnancy tests have been found to be highly reliable. However, the versions of these tests performed by qualified laboratory technologists are considered to be definitive. Often, such a test will produce positive results before a woman experiences symptoms or before a doctor's exam reveals signs of pregnancy.
- Ectopic pregnancy
- A pregnancy that develops outside of the mother's uterus. Ectopic pregnancies often cause severe pain in the lower abdomen and are potentially life-threatening because of the massive blood loss that may occur as the developing embryo/fetus ruptures and damages the tissues in which it has implanted.
- In humans, the developing individual from the time of implantation to about the end of the second month after conception. From the third month to the point of delivery, the individual is called a fetus.
- A chemical produced by a specific organ or tissue of the body that is released into the bloodstream in order to exert an effect in another part of the body.
- Human chorionic gonadotropin (hCG)
- A hormone produced by the placenta of a developing pregnancy.
- Hydatidform mole
- A rare, generally benign grape-like mass that grows in the uterus from the remains of an abnormally developed embryo and surrounding tissue. In extremely rare cases, the mole develops into a choriocarcinoma, a malignant tumor whose cells can invade the wall of the uterus.
- The attachment of the fertilized egg or embryo to the wall of the uterus.
- Menstrual cycle
- A hormonally regulated series of monthly events that occur during the reproductive years of the human female to ensure that the proper internal environment exists for fertilization, implantation, and development of a baby. Each month, a mature egg is released from the follicle of an ovary. If an egg is released, fertilized, and implanted, the lining of the uterus continues to build. If fertilization and/or implantation does not occur, the egg and all of the excess uterine lining are shed from the body during menstruation.
- Loss of the embryo or fetus and other products of pregnancy before the middle of the second trimester. Often, early in a pregnancy, if the condition of the baby and/or the mother's uterus are not compatible with sustaining life, the pregnancy stops, and the contents of the uterus are expelled. For this reason, miscarriage is also referred to as spontaneous abortion.
- The organ that unites the developing new life (first called an embryo and later a fetus) to the mother's uterus. The placenta produces hCG, among other hormones, to help maintain the pregnancy. After delivery, the placenta, known at this point as afterbirth, is expelled.
For Your Information
- Cunningham, F. Gary, ed. Williams Obstetrics, 20th ed. Stanford: Appleton and Lang, 1977.
- Bastian, L.A., et al. "Is This Patient Pregnant?" JAMA: The Journal of the American Medical Association 278 no. 7: 586-591.
- Peredy, T.R., and R.D. Powers. "Bedside Diagnostic Testing of Body Fluids." American Journal of Emergency Medicine 15 no. 4: 404-405.
- "Pregnancy Tests: They Can't Get Much Simpler Than This." Consumer Reports (October 1996): 48-49.
- "Reliable Ways to Find Out if You're Pregnant or Ready to Conceive." Available at Mayo Clinic Health Oasis. (28 Feb. 1996). http://www.mayo.ivi.com.
Gale Encyclopedia of Medicine. Gale Research, 1999.