Every parent-to-be spends happy hours envisioning a healthy baby. But these daydreams are often accompanied by moments of worry - what if the baby has a serious or untreatable health problem? What would I do? Would it be my fault?
Concerns like these are completely natural, and some may be exaggerated by news stories about genetics and genetic testing, which promises that someday parents may be able to pick only "desirable" traits in their unborn children. With all the medical information available, you may feel as though you have to undergo a battery of prenatal tests to make sure your baby is healthy.
Prenatal tests can serve a useful function in terms of identifying, and sometimes treating, health problems that could endanger both you and your unborn child. However, they have limitations. As an expecting parent, you should take the time to educate yourself about these tests and to think about what you would do if a health problem is detected.
Why Are Prenatal Tests Performed?
Prenatal tests do several different things. They can identify:
- treatable health problems in the mother that can affect the baby's health
- characteristics of the fetus, including size, sex, age, and placement in the uterus
- the chance that a baby has certain congenital, genetic, or chromosomal problems
- certain types of fetal abnormalities, including heart problems
The last two items on this list may seem the same, but there's a key difference. Some prenatal tests are screening tests and only reveal the possibility of a problem existing - they don't provide a definitive diagnosis. Other prenatal tests are diagnostic in nature, which means they can determine with a fair degree of certainty whether a fetus has a specific problem. Many women whose screening tests reveal the possibility of an abnormality have healthy babies, but in the interest of making the more specific determination, the screening test may be followed by a more invasive - and riskier - diagnostic test.
The issue of prenatal testing is further complicated by the fact that approximately 250 birth defects can be diagnosed in an unborn fetus - many more than can be treated or cured. This raises the question of what a parent will do once a defect or problem is detected.
What Do Prenatal Tests Find?
Among other things, routine prenatal tests can determine key things about the mother's health including her blood type, whether she suffers from gestational diabetes, her immunity to certain diseases, and whether she has a sexually transmitted disease (STD) or cervical cancer. All of these conditions can affect the health of the fetus.
Prenatal tests also can determine things about the fetus' health including whether it's one of the 2% to 3% of babies in the United States that the American College of Obstetricians and Gynecologists (ACOG) says have major congenital birth defects. There are different categories of defects screened by prenatal tests, including:
Dominant gene disorders
In dominant gene disorders, there's a 50-50 chance a child will inherit the gene from the affected parent and have the disorder. Dominant gene disorders include:
- achondroplasia
, a rare abnormality of the skeleton causing shorter-than-normal arms and legs
- Huntingdon disease
, a disease of the nervous system that causes neurologic deterioration affecting people in their 30s and 40s
Recessive gene disorders
Because there are so many genes in each cell, everyone carries some abnormal recessive genes, but most people don't have a defect because the normal gene overrules the abnormal one. But if a fetus has a pair of abnormal recessive genes (one from each parent), the child will have the disorder. It's more likely for this to happen in children born to certain ethnic groups or to parents who are blood relatives. Recessive gene disorders include:
- cystic fibrosis
(most common among people of northern European descent), a disease that causes the respiratory system to produce thick mucus that clogs the lungs
- sickle cell disease
(most common among people of African descent), a disease where red blood cells form a "sickle" shape, rather than the typical donut shape, get caught in blood vessels, and cut off oxygen to tissues
- Tay-Sachs disease
(most common among people of European [Ashkenazi] Jewish descent), a disorder causing mental retardation, blindness, seizures, and death
- beta thalassemia
(most common among people of Mediterranean descent), a disorder causing anemia
X-linked disorders
These disorders are determined by genes on the X-chromosome of the pair of chromosomes that determine sex. These disorders are much more common in boys because the pair of sex chromosomes in males contains only one X-chromosome (the other is a Y-chromosome). If the disease gene is present on the one X-chromosome, the X-linked disease shows up because there's no other paired gene to 'overrule' the disease gene. Hemophilia is one such X-linked disorder; people who have it lack a crucial clotting agent in their blood.
Chromosomal disorders
Some chromosomal disorders are inherited, but most are caused by a sporadic error in the genetics of the egg or sperm. The chance of a child having these disorders increases with the age of the mother. For example, according to the ACOG, one in 1,667 live babies born to 20-year-olds have Down syndrome; that number changes to one in 378 for 35-year-olds and one in 106 for 40-year-olds. Down syndrome causes mental retardation and physical defects.
Multifactorial disorders
This final category includes disorders that are caused by a mix of genetic and environmental factors. The frequency of these disorders varies from country to country; some can be detected during pregnancy. Multifactorial disorders include neural tube defects, which occur when the tube enclosing the spinal cord doesn't form properly. Neural tube defects include spina bifida and anencephaly. Spina bifida is also called "open spine" and occurs when the lower part of the neural tube doesn't close during embryo development, leaving the spinal cord and nerve bundles exposed. Anencephaly occurs when the brain and head don't develop properly, with the top half of the brain being completely absent. Neural tube defects have been associated with inadequate intake of folic acid during the early part of pregnancy, among other factors.
Other multifactorial disorders include:
Who Has Prenatal Tests?
Certain prenatal tests are considered routine - that is, almost all pregnant women receiving prenatal care get these tests. Others are recommended only for certain women, especially those who have what are known as high-risk pregnancies. George Macones, MD, the director of maternal/fetal medicine and director of obstetrics for the University of Pennsylvania Health System in Philadelphia, typically recommends nonroutine tests to women who:
- are age 35 or older
- have had a premature baby
- have had a baby with a birth defect - especially heart or genetic problems
- have high blood pressure, diabetes, lupus, asthma, or a seizure disorder
- have or whose partner has an ethnic background where genetic disorders are common
- have or whose partner has a family history of mental retardation
Dr. Macones is also careful to point out that although he recommends these tests, ultimately it's up to the mother if she wants to have them. "I spend a lot of time talking to parents before the mother undergoes, for example, amniocentesis," he says. "Patients need to be educated before they make decisions."
In addition to talking to their obstetricians, women who have a family history of genetic problems in their families (or whose partners do) may want to consult with a genetic counselor who can help them construct a family tree going back as far as three generations.
To decide which tests are right for you, it's important to carefully discuss with your doctor what these tests are supposed to measure, how reliable they are, the potential risks, and your options and plans if the results indicate a disorder or defect.
Routine Prenatal Tests
On your first visit to the doctor for prenatal care, you'll undergo certain tests regardless of your age or genetic background.
Blood tests determine your blood type and Rh factor. If your blood is Rh positive and your partner's is Rh negative, you may develop antibodies that prove dangerous to your fetus. This can be treated through a course of injections. Blood tests also measure the level of iron in your blood and check for hepatitis B, syphilis, and HIV. You'll also be tested to see whether you're immune to rubella (German measles).
Urine tests check for kidney infections and signs of gestational diabetes and pregnancy-induced high blood pressure (which can cause a specific protein to show in the urine).
Cervical tests check for STDs (such as chlamydia and gonorrhea), cervical cancer, and Group B streptococcus infection. Group B streptococcus, which are bacteria that are not transmitted sexually, can cause serious infections in newborns.
Around the 16th to 18th week of pregnancy, most women will have a maternal blood screening test performed. Also known as a "triple-marker" test, it measures the levels of a protein produced by the fetus and two pregnancy-produced hormones in the mother's blood. This test can reveal the chances that a mother is carrying a fetus with neural tube defects or Down syndrome.
It's at this point that most women will also have their first ultrasound test, which helps the doctor identify the position of the baby and its gender as well as helping to detect Down syndrome, other chromosome abnormalities, structural defects such as spina bifida and anencephaly, and inherited metabolic disorders.
Around the 24th week of pregnancy, an additional screening for gestational diabetes may be performed.
Chart of Prenatal Tests
This chart includes some tests that are now performed almost routinely in the United States and those that are performed only in high-risk pregnancies or if the doctor suspects an abnormality in the fetus.
Chorionic Villus Sampling (CVS)
- Why Is This Test Performed?
Chorionic villi are microscopic finger-like projections that make up the placenta. They develop from the same fertilized egg as the fetus and reflect the fetal genetic makeup. This newer alternative to amniocentesis removes some of the chorionic villi and tests them for chromosomal abnormalities, such as Down syndrome. Its advantage over amniocentesis is that it can be performed earlier, allowing more time for expectant parents to receive counseling and make decisions.
- Should I Have This Test?
If you are older than age 35, have a family history of genetic disorders (or a partner who does), or have a previous child with a birth defect, your doctor may recommend this test for you. ACOG says this test carries between a 0.5% and 1% risk of miscarriage. It may cause intrauterine growth retardation, prematurity, or early labor. Other risks include infection and spotting or bleeding (this is more common in the transcervical test).
- When Should I Have This Test?
10 to 12 weeks
- How Is the Test Performed?
This test is performed in one of two ways:
- Transcervical
Using ultrasound as a guide, a thin tube is passed from the vagina into the cervix. Gentle suction removes a sample of tissue from the chorionic villi. No anesthetic is used, although some women do experience a pinch and cramping.
- Transabdominal
A needle is inserted through the abdominal wall - this minimizes chances of intrauterine infection, and in women whose uterus is tipped, reduces the chance of miscarriage. After the sample is taken, the doctor will check the fetal heart rate. You should rest for a couple hours afterward.
- When Are the Results Available?
Less than 1 week for Down syndrome or about 2 weeks for a thorough analysis
Maternal Blood Screening
- Why Is This Test Performed?
Doctors use this to test the mother's blood only for alpha-fetoprotein (AFP). AFP is the protein produced by the fetus, and it appears in varying amounts in the mother's blood and the amniotic fluid at different times during pregnancy. A certain level in the mother's blood is considered normal, but higher or lower levels may indicate a problem. This test has been expanded, however, to include two pregnancy hormones called estriol and human chorionic gonadotropin (HCG), which is why it's now sometimes referred to as a "triple screen." This test calculates a woman's individual risk of birth defects based on the levels of the three (or more) substances plus her age, weight, race, and whether she has diabetes requiring insulin treatment. It's important to note that this screening test determines risk only - it doesn't diagnose a condition.
- Should I Have This Test?
All women are offered this test. Remember that this is a screening, not a definite test - it indicates whether a woman is likely to be carrying an affected fetus. It's also not foolproof - spina bifida may go undetected, and some women with high levels have been found to be carrying a healthy baby. Further testing is recommended to confirm a positive result.
- When Should I Have This Test?
16 to 18 weeks
- How Is the Test Performed?
Blood is drawn from the mother.
- When Are the Results Available?
3 to 5 days, although it may take up to a week or 2
Amniocentesis
- Why Is This Test Performed?
This test is used most often to detect Down syndrome and other chromosome abnormalities, structural defects such as spina bifida and anencephaly, and inherited metabolic disorders. Other common birth defects, such as heart disorders and cleft lip and palate, can't be determined using this test. Late in the pregnancy, this test can reveal if a baby's lungs are strong enough to allow the baby to breathe normally after birth, which can help the doctor make decisions about inducing labor.
- Should I Have This Test?
If you are older than age 35, have a family history of genetic disorders (or a partner who does), or have a previous child with a birth defect, your doctor may recommend this test for you. This test can be very accurate - close to 100% - but only certain disorders can be detected. According to the Centers for Disease Control and Prevention (CDC), the rate of miscarriage with this procedure is between one in 400 and one in 200. The procedure also carries a lower risk of uterine infection (less than one in 1,000), which can cause miscarriage.
- When Should I Have This Test?
16 to 18 weeks
- How Is the Test Performed?
A needle is inserted through the abdominal wall into the uterus, removing some of the amniotic fluid. A local anesthetic may be used. Some women report they experience cramping when the needle enters the uterus or pressure while the doctor retrieves the sample. The doctor will check the fetus' heartbeat after the procedure to make sure it's normal. Most doctors recommend rest for a couple hours after the procedure. One ounce of fluid is withdrawn and sent to a lab for testing. The cells in the fluid are grown in a special culture and then analyzed. The specific tests conducted depend on personal and family medical history.
- When Are the Results Available?
Up to 1 month (with the possibility that the lab will ask for a repeat), but tests of lung maturity are available immediately
Ultrasound
- Why Is This Test Performed?
In this test, sound waves are bounced off the baby's bones and tissues to construct an image showing the baby's shape and position in the uterus. Ultrasounds were once used only in high-risk pregnancies but have become so common that they are often part of routine prenatal care. In addition to showing the fetus' age, rate of growth, position, movement, breathing, and heart rate, it shows the number of fetuses and the amount of amniotic fluid in the uterus. The test is used most often to detect Down syndrome and other chromosome abnormalities, structural defects such as spina bifida and anencephaly, and inherited metabolic disorders. Congenital heart defects, gastrointestinal and kidney malformations, and cleft lip or palate may also be determined. Ultrasound can indicate the position of the placenta in late pregnancy (which may be blocking the baby's way out of the uterus). They can be used to detect pregnancies outside the uterus and they can guide other tests by showing placement of the fetus.
- Should I Have This Test?
Most women have this test. You may want to ask your doctor about ultrasound. Find out if it's the most appropriate test for you and discuss the risks and benefits. There are no proven side effects of ultrasound to the mother or fetus, although it's still being studied.
- When Should I Have This Test?
16 to 18 weeks (it can be done earlier and later if necessary, especially if your doctor wants to monitor fetal growth)
- How Is the Test Performed?
Women need to have a full bladder for a transabdominal ultrasound to be performed in the early months - you may be asked to drink a lot of water and not urinate. You'll lie on an examining table, and your abdomen will be coated with a special ultrasound gel. A technician will pass a transducer back and forth over your abdomen, while a computer translates the waves into an image called a sonogram. You may want to ask to have the picture interpreted for you, even in late pregnancy - it often doesn't look like a baby to the untrained eye. Sometimes, if the radiologist isn't getting a good enough image from the ultrasound, he or she will determine that a transvaginal ultrasound is necessary. This is especially common in early pregnancy. For this procedure, your bladder should be empty. Instead of a transducer being moved over your abdomen, the high-frequency waves will be emitted by a probe called an endovaginal transducer, which is placed in your vagina. This technique often provides improved images of the uterus and ovaries. A radiologist, who is a physician experienced in obstetric ultrasound, will analyze the images and send a signed report with his or her interpretation to your doctor.
- When Are the Results Available?
Immediately (but a full evaluation may take up to 1 week)
Glucose Screening
- Why Is This Test Performed?
Glucose screening checks for gestational diabetes, a short-term form of diabetes that develops in some women during pregnancy. Gestational diabetes occurs in 1% to 3% of pregnancies and can cause health problems for the baby.
- Should I Have This Test?
Most women have this test at 24 weeks, but if you've had high sugar in two routine urine tests, your doctor may order it earlier.
- When Should I Have This Test?
24 weeks
- How Is the Test Performed?
Blood is drawn after you've consumed a sugary drink. If the reading is high, you'll have a glucose-tolerance test, which means you'll drink a glucose solution on an empty stomach and have your blood drawn once every hour for 3 hours.
- When Are the Results Available?
Immediately
Nonstress Test
- Why Is This Test Performed?
If you've gone beyond your due date, this test uses external fetal monitoring to determine fetal movement. This test is used mostly in high-risk pregnancies or when the doctor is uncertain of fetal movement. The nonstress test can help a doctor make sure that the baby is receiving enough oxygen and that the nervous system is responding. A nonresponsive baby doesn't necessarily mean that the baby is in danger.
- Should I Have This Test?
Your doctor may recommend this if you have a high-risk pregnancy or if you have a low-risk pregnancy and you're past your due date.
- When Should I Have This Test?
1 week after due date
- How Is the Test Performed?
The doctor will measure the response of the fetus' heart rate to each movement the fetus makes as reported by the mother or observed by the doctor on an ultrasound screen. If the fetus doesn't move during the test, he may be asleep and the doctor may use a buzzer to wake him.
- When Are the Results Available?
Immediately
Contraction Stress Test
- Why Is This Test Performed?
This test stimulates the uterus with pitocin, a synthetic form of oxytocin (a hormone secreted during childbirth), and determines the effect of contractions on fetal heart rate. It's usually recommended when a nonstress test indicates a problem and can determine whether the baby's heart rate remains stable during contractions.
- Should I Have This Test?
This test is usually ordered if the nonstress test indicates a problem. It does have a high false-positive rate, though, and can cause labor to be induced prematurely.
- When Should I Have This Test?
After 40 weeks
- How Is the Test Performed?
Mild contractions are brought on either by injections of pitocin or by squeezing the mother's nipples (which causes oxytocin to be secreted). The fetus' heart rate is then monitored.
- When Are the Results Available?
Immediately
Percutaneous Umbilical Vein Sampling (PUVS)
- Why Is This Test Performed?
This test obtains fetal blood by guiding a needle into the umbilical vein. It's primarily used in addition to ultrasound and amniocentesis if your doctor needs to quickly check your baby's chromosomes for defects or disorders or if your doctor is concerned that your baby may be anemic. The advantage to this test is its speed. There are situations (such as when a fetus shows signs of distress) in which it's helpful to know whether the fetus has a fatal chromosomal defect. If the fetus is suspected to be anemic, this test is the only way to confirm this, and it also allows transfusion while the needle is in place.
- Should I Have This Test?
This test is used late in a pregnancy after an abnormality has been noted on an ultrasound, when amniocentesis results are not conclusive, if the fetus may have Rh disease, or if you've been exposed to an infectious disease that could potentially affect fetal development.
- When Should I Have This Test?
Between 18 and 36 weeks
- How Is the Test Performed?
A fine needle is passed through your abdomen and uterus into the fetal vein in the umbilical cord and blood is withdrawn for testing.
- When Are the Results Available?
3 days
Talking to Your Doctor About Prenatal Tests
Prenatal tests can be stressful, and because many aren't definitive, even a negative result may not ease any anxiety you may be experiencing. Because many women who have abnormal tests end up having healthy babies and because many of the problems that are detected cannot be treated, some women decide to forgo some of the testing.
One important thing to consider is what you will do in the event that a birth defect is discovered. Implicit in much of this testing is that you can make a decision to terminate the pregnancy based on the results. Your obstetrician or a genetic counselor can help you establish priorities, give you facts, and discuss your options.
It's important to remember that tests are offered to women - they are not mandatory. You should feel free to ask your doctor why he or she is ordering a certain test, what the risks and benefits of the test are, and most importantly, what the results will - and won't - tell you.
"The women who are most stressed about tests are the women who don't understand what the results are going to be like," Dr. Macones says. "It's important that doctors educate the patient - if not right in the office, then by giving her literature about each type of test."
If you think that your doctor isn't answering your questions adequately, you should say so. You don't have to accept the answer "I do this test on all of my patients." Questions to ask include:
- How much will the test cost? Will it be covered by insurance?
- What do I need to do to prepare?
- How long before I get the results? How accurate is this test?
- What are you looking to get from these test results? What do you hope to learn?
- Is the procedure painful? Is it dangerous to me or the fetus? Do the potential benefits outweigh the risks?
- What could happen if I don't undergo this test?
Preventing Birth Defects
The best thing that mothers-to-be can do to avoid birth defects is to make sure they take care of their bodies during pregnancy by:
- not smoking (and avoiding second-hand smoke)
- avoiding alcohol
- eating a healthy diet and taking prenatal vitamins
- getting exercise and plenty of rest
- getting prenatal care
Dr. Macones also points out that there are women who should talk to their doctor before becoming pregnant (including women with diabetes and seizure disorders) to obtain genetic counseling.
Reviewed by: George Macones, MD
Date reviewed: August 2000
Source: www.KidsHealth.org