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Antenatal Testing

Definition

Antenatal testing describes procedures performed during pregnancy to detect health problems in the growing fetus; establish characteristics such as fetal age, sex, or weight; or diagnose any material conditions that may affect fetal development.

Purpose

Antenatal tests and exams are important tools for protecting the health of a pregnant woman and her developing child. Various tests are administered over the course of pregnancy to determine if the mother has any health conditions that may interfere with normal development of the fetus or if the fetus has any health conditions that may affect the baby's quality of life. Often, families will use information provided by the tests to prepare for the baby's birth and make arrangements for special care if needed or make the decision to terminate the pregnancy. Physicians also use antenatal tests to determine various characteristics of the fetus, such as gestational age, size, and position in the uterus, or to verify the presence of multiple fetuses.

Description

Women who become pregnant may undergo tests at any stage in their pregnancy: during the first trimester (weeks one through 12), second trimester (weeks 13–26), or third trimester (weeks 27–40+). What tests are ordered depends on the stage of pregnancy, the age and health of the mother, the medical history of both parents, and the family's background or ethnicity. There are two distinct types of antenatal tests:

In the early stages of pregnancy, physicians may order blood or urine tests to screen for possible disorders or infections that could affect the growing fetus. The tests may also be ordered in later stages if the pregnant woman comes in contact with an infectious agent or develops symptoms of infection. In many cases, complications can be avoided if early diagnosis is made and treatment initiated. Examples of conditions that are commonly screened for with blood and/or urine tests include:

A multiple marker test or triple screen is used to determine if a fetus is at an increased risk of having certain congenital abnormalities. The test has a high rate of false-positives; as few as 10 percent of women with abnormal results go on to have babies with congenital defects. The purpose of the test is to determine if further testing (such as ultrasound or amniocentesis ) is warranted. The test requires a sample of maternal blood, typically taken during the fifteenth and twentieth week of pregnancy, and measures the level of certain pregnancy hormones.

Ultrasound is a device that records sound waves as they bounce off the developing fetus and create an image that is projected onto a large computer screen. A physician may order an ultrasound scan to listen for a fetal heartbeat, determine a woman's precise due date, or check for twins , among other uses. Also known as a sonogram, the procedure takes only a few minutes, is safe and painless, and usually is covered by health insurance.

During the procedure, an ultrasound technician asks the pregnant woman to remove her clothes and change into a gown. The technician may rub some gel on the woman's fundus (lower abdomen), which helps the hand-held device pick up sound waves. In certain cases, the technician may insert a plastic probe into the woman's vaginal canal to get a clearer picture of the fetus. Early in pregnancy, the test may need to be done with a full bladder.

Pregnant women will often have their first ultrasound between eight and 12 weeks of gestation. In normal cases, the technician is able to identify a fetal heartbeat, which appears as a flashing light on the screen. Closer to the due date, physicians use ultrasound to make sure the fetus is in the correct head-first position to exit the birth canal, to assess the fetus for certain birth defects, and to determine the sex of the fetus if the parents desire.

Between ten and 14 weeks of gestation, ultrasonography may be used to measure a small collection of fluid beneath the skin at the back of the neck. Called nuchal translucency, the measurement tends to be larger in fetuses with genetic abnormalities such as Down syndrome , trisomy 13, trisomy 18, Turner syndrome , and triploidy. A particular neck measurement combined with maternal age as an indicator (e.g. the incidence of the disorder increases in proportion to the age of the mother) has been shown to correctly diagnose Down syndrome in 75 to 80 percent of cases; this number increases to 90 percent if the procedure is combined with the multiple marker test.

Amniocentesis is a more invasive test that carries a higher risk of complications than blood tests or ultrasonography, but is able to determine more precisely the presence of certain birth defects. It is also used to determine the level of maturity of the baby's lungs, of particular interest if the baby will be delivered prematurely. During amniocentesis, a doctor inserts a thin needle through a woman's abdomen and into the uterus. Using ultrasound as a guide, the doctor uses the needle to withdraw a sample of fluid from the amniotic sac. Afterward, tiny cells shed by the fetus can be studied in the laboratory; scientists can analyze the samples to determine if the fetus has certain genetic conditions. Amniocentesis is typically performed during the second trimester of pregnancy and particularly in mothers over the age of 40.

Chorionic villus sampling (CVS) is a procedure that allows for prenatal diagnosis during the first trimester (generally between ten and 12 weeks of gestation, during the embryonic stage of development). The test involves taking a small sample of cells from the placenta with a needle through the abdominal wall or a small tube (catheter) through the cervix. The procedure is guided by ultrasound. The cells are then analyzed in a laboratory for the presence of genetic abnormalities.

Risks

The risks associated with antenatal testing depend on the specific procedure being performed:

Parental concerns

KEY TERMS

Alpha fetoprotein test —A screening blood test that can be done after the sixteenth week of pregnancy to evaluate the possibility of spina bifida and other neural tube defects in the fetus.

Amniocentesis —A procedure performed at 16–18 weeks of pregnancy in which a needle is inserted through a woman's abdomen into her uterus to draw out a small sample of the amniotic fluid from around the baby for analysis. Either the fluid itself or cells from the fluid can be used for a variety of tests to obtain information about genetic disorders and other medical conditions in the fetus.

Breech position —The foot-down or bottom-down position of a fetus just before delivery.

Group B streptococcus —A serotype of streptococcus, Streptococcus agalactiae, which is beta hemolytic and can cause neonatal sepsis, pneumonia, or meningitis if present in the birth canal at the time of delivery especially when the delivery is difficult.

Hemoglobin —An iron-containing pigment of red blood cells composed of four amino acid chains (alpha, beta, gamma, delta) that delivers oxygen from the lungs to the cells of the body and carries carbon dioxide from the cells to the lungs.

Meningitis —An infection or inflammation of the membranes that cover the brain and spinal cord. It is usually caused by bacteria or a virus.

Ultrasonography —A medical test in which sound waves are directed against internal structures in the body. As sound waves bounce off the internal structure, they create an image on a video screen. Ultrasonography is often used to diagnose fetal abnormalities, gallstones, heart defects, and tumors. Also called ultrasound imaging.

Resources

Seashore, Margretta R. "Genetic Counseling." In Cecil Textbook of Medicine. Edited by Lee Goldman and J. Claude Bennett. Philadelphia: Saunders, 2000.

Simpson, Joe Leigh. "Diagnostic Procedures for Prenatal Genetic Diagnosis." In Obstetrics: Normal and Problem Pregnancies. Edited by Steve G. Gabbe et al. New York: Churchill Livingstone, 2002.

Andrews, Janet I., Daniel J. Diekema, and Jerome Yankowitz. "Prenatal Testing for Infectious Disease." Clinics in Laboratory Medicine 23, no. 2 (June 2003): 295–315.

Bubb, Jennifer A., and Anne L. Matthews. "What's New in Prenatal Screening and Diagnosis?" Primary Care 31, no. 3 (September 2002): 561.

American College of Obstetricians and Gynecologists. 409 12th St., SW, PO Box 96920, Washington, DC 20090–6920. Web site: http://www.acog.org.

March of Dimes Birth Defects Foundation. 275 Mamaroneck Ave., White Plains, NY 10605. Web site: http://www.modimes.org.

Singh, Daljit, Jai Rup Singh, and Vanita Kumar. "Prenatal Diagnosis for Congenital Malformations and Genetic Disorders." eMedicine , July 29, 2004. Available online at http://www.emedicine.com/oph/topic485.htm (accessed December 23, 2004).

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