Pregnancy can be an exciting time for expectant parents. It can also be a time of concern over the health of the baby. Medical technology has led to the development of many prenatal screening tools, which are used to identify mothers and babies who are at an increased risk for developing a disease or condition.
Prenatal screening results do not give a “yes” or “no” answer about the presence of a disease or whether it will develop, only if it is more or less likely. Many women who test positive for a screening do not actually have the condition. The screening is a way to rule out people who are at low risk and identify those at high risk.
Birth defects are physical abnormalities that may be caused by inherited genes, genetic mutations, or environmental factors. They cause deformities or disabilities that cannot always be treated or cured. Birth defects that are most often identified by screenings are chromosome abnormalities like ]]>Down's syndrome]]> and neural tube defects like ]]>spina bifida]]>.
Routine Screenings to Monitor Health of the Mother
A woman’s health directly impacts the health of her fetus. At the first prenatal care appointment, the doctor will review the mother’s family and personal health history and offer routine screenings so issues that could threaten the fetus can be controlled or anticipated.
Routine screenings may include:
- Blood pressure reading
- Urine tests (eg, to detect ]]>urinary tract infection]]>, protein in the urine, and signs of ]]>gestational diabetes]]>)
- Lab analysis for infections, such as ]]>gonorrhea]]> , ]]>chlamydia]]> , and ]]>group B streptococcal disease]]>
- Blood tests for:
Screenings to Monitor Health of the Fetus
Screenings also enable doctors to monitor the health of a fetus during development. In many cases, when screenings produce abnormal results, subsequent testing will be recommended. Usually a combination of two or more of these screenings will produce the most reliable results. Some screenings are routine, while others are prompted by certain criteria, such as a mother’s age or abnormal results of a routine screening.
Examples of screenings include:
Maternal Serum Alpha-Fetoprotein (MSAFP)
MSAFP, known as triple screen, is a blood test designed to identify neural tube defects. The test detects alpha-fetoprotein (AFP) and also measures the levels of two pregnancy hormones―estriol and human chorionic gonadotropin (HCG).
Elevated levels of AFP can indicate neural tube defects, as well as the presence of twins or a miscalculated date of conception. Low levels indicate possible Down's syndrome or other chromosomal abnormalities. This screening test may be done at 15-20 weeks. MSAFP is not as accurate in identifying chromosomal abnormalities, but about 75% to 90% of babies with neural tube defects can be identified with this test.
Another option is the quad screen, which tests for an additional protein called inhibin-A. This test may be better able to identify Down's syndrome.
]]>Amniocentesis]]> involves obtaining a sample of amniotic fluid by inserting a thin needle into the uterus. Chromosomes in the fluid are evaluated for Down's syndrome, genetic birth defects, and other abnormalities, as well as the baby's sex (if the parents would like to know).
Amniocentesis is usually done between the 15th and 16th week. If the test is done earlier in the pregnancy, there could be a greater risk of miscarriage or other complications. The doctor may recommend amniocentesis if the mother is aged 35 or older, there is a family history of genetic abnormalities, or there are abnormal results from the other screenings.
Chorionic Villi Screening (CVS)
CVS involves removing a sample of the placenta to detect chromosomal abnormalities. The test provides more information than amniocentesis, and it may be done earlier in the pregnancy (usually between 10-12 weeks), which provides additional time for counseling and decision-making. It has a slightly higher risk of damage to the fetus. Because CVS does not test amniotic fluid, other tests should be done to check for neural tube defects.
An ]]>ultrasound]]> uses sound waves to produce a picture of the baby on a computer screen. It can determine the age of a fetus. The test can also identify twins, evaluate fetal structure, study breathing, and measure amount of amniotic fluid. An ultrasound may be done at anytime during the pregnancy. In the first trimester it can be done to make sure the age of the fetus is known. In the second trimester it is often done to evaluate fetal structure.
Non-stress Test (NST)/Biophysical Profile (BPP)
These tests are sometimes done with high-risk pregnancies to monitor the baby’s heartbeat and observe fetal activity, muscle tone, and breathing, as well as amniotic fluid volume.
Nuchal Translucency (NT)
This screening assesses a baby’s risk for Down's syndrome in the first trimester. It uses ultrasound to measure the space in the tissue at the back of the fetus’ neck, where fluid tends to accumulate if Down's syndrome is present. The results of this test can indicate the need for further testing.
Percutaneous Umbilical Blood Sampling
This is the most accurate way to diagnose Down's syndrome, but it carries the highest risk of miscarriage. It is used to confirm positive results of a CVS or amniocentesis and cannot be done until the 18th to 22nd week of pregnancy.
Psychological Aspects of Screening
Screenings can be stressful for the parents, especially when results are abnormal and difficult decisions have to be made based on the screening information. While the screenings may help prepare parents for unexpected outcomes of the pregnancy, there may be no treatment that can be given while the fetus is still in utero.
Nonetheless, many who test positive for a screening will test negative in follow-up diagnostic tests.
Genetic counseling, an important component of prenatal screenings, can assist parents in understanding what the results mean, evaluating treatment options, and considering the possible need for further testing.
American Pregancy Association
March of Dimes
The Society of Obstetricians and Gynaecologists of Canada
Women's Health Matters
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Last reviewed July 2010 by ]]>Brian Randall, MD]]>
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