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Using special tests, it is now possible to find out a great deal about the baby while it is still in the womb. Certain tests in the first half of the pregnancy, for example, can detect serious abnormailities so that, if the parents can decide on what to do.

What is the nuchal translucency screening test?

This prenatal screening test (also called the nuchal fold scan) uses ultrasound to measure the clear ("translucent") space in the tissue at the back of your developing baby's neck. That measurement can help your healthcare practitioner assess your baby's risk for Down syndrome (DS) and other chromosomal abnormalities as well as major congenital heart problems.

Babies with abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this clear space to be larger. While the nuchal translucency screening test won't give you the definite diagnosis you'd get from more invasive tests like CVSand amniocentesis, it can help you decide whether you want to undergo diagnostic testing. And unlike diagnostic tests, it's painless and involves no risk to you or your baby.

This test has been performed in the United States since 1995, mostly at large medical centers. But it's becoming more widely available as more ultrasound technicians (sonographers) and doctors get trained and certified to do the test.

Sonographers and doctors need special training and high-quality equipment to perform it correctly, and they must be certified by the Fetal Medicine Foundation in London, the organization that sets the international standards and provides the software that allows a doctor to evaluate your risk. If you're interested, ask your healthcare practitioner or genetic counselor whether it's offered in your area.

How is the screening done?

The nuchal translucency screening must be done when you're between 11 and 14 weeks pregnant. (The last day you can have it done is the day you turn 13 weeks and 6 days pregnant.)

The sonographer first confirms your baby's gestational age by measuring your baby from crown to rump to see if he's about the size he should be for his age. Then she positions the sensor, called the transducer, over your abdomen so that your baby's nuchal fold area shows up on the monitor, and measures the thickness of it on the screen with calipers.

The doctor will put that nuchal fold measurement into a formula along with your age and your baby's gestational age, so that your baby's chances of having a chromosomal abnormality, based on statistical probability, can be computed. Researchers have measured the nuchal folds of thousands of babies between 11 and 14 weeks of pregnancy. And because a baby's nuchal fold will normally get a bit thicker with each day of gestation, they've been able to establish what an average or "normal" thickness would be for each day during those three weeks.

They've also figured out the statistical relationship between this measurement, the baby's age, the mother's age, and the likelihood that the baby will be born with certain abnormalities. In general, the thicker the fold at a given gestational age, the higher the chance of a chromosomal problem.

You may get the results right away, or you may have to wait up to ten days if the doctor has to send the data to a processing center. You'll want to talk with your practitioner or a genetic counselor about how to interpret the results, because unless you have a lot of experience with statistical analyses and evaluating medical tests, they can be confusing. And even if you're a wizard with statistics, you'll need help understanding your options.

What do the results mean?

You'll get your results in the form of a ratio that expresses your baby's chances for having a chromosomal defect (based on your age, his age, and his nuchal fold measurement). For example, if you're going to be 35 when you deliver, your baby's average risk for Down syndrome is 1 in 270. (This risk gets higher as you get older.) That means that one in every 270 babies born to 35-year-old women will have DS. One way to think about what this means is to picture yourself in a room with 270 other 35-year-old pregnant women — statistically, only one of you would be carrying a baby with Down syndrome.

If you're 35 and your baby's nuchal fold measurement is average for his age, your baby's risk for DS stays the same: 1 in 270. If it's thicker than the average, he has a higher risk for an abnormality. If his nuchal fold is thinner than the average, his risk is lower.

Remember that this test doesn't directly test for chromosomal problems — it just gives you a better idea of your baby's statistical likelihood of having a problem. A normal result (sometimes called "screen negative") isn't a guarantee that your baby is normal, but it suggests that a chromosomal problem is unlikely. And an abnormal result (sometimes called "screen positive") doesn't mean that your baby has a chromosomal problem — just that he's more likely to have one. In fact, most "screen positive" babies turn out to be normal.

With the help of your practitioner or a genetic counselor, you'll then want to decide whether the results indicate a high enough risk that you want to have more testing to get a definitive diagnosis — that is, to see whether your baby really does have a chromosomal defect. Individual parents-to-be have different feelings about what's an "acceptable" risk. Tests that can diagnose a chromosomal defect include chorionic villus sampling (CVS) and amniocentesis.

(If your baby's nuchal fold measurement is above the 99th percentile for his gestational age, he's also considered to have an increased risk for major congenital heart disease, so you'll be given another screening test called a fetal echocardiogram. If your baby has heart disease, he'll need to be monitored with regular ultrasounds and delivered at a medical center equipped to handle his condition.)

Can you give an example of someone's results?

Here's an example of how it might go: A 40-year-old pregnant woman starts out with a 1 in 77 chance of having a baby with Down syndrome. She comes in for a nuchal fold scan at 11 weeks and 2 days, and the sonographer finds that her baby's nuchal fold is very thin for his gestational age. When the doctor feeds the results (the nuchal measurement, the baby's age, and the mother's age) into the risk assessment program, he finds that her baby's risk is now down to 1 in 761. Her baby's risk for trisomy 18, another abnormality, is even lower — 1 in 10,000. The woman is relieved and decides that since her risk seems low enough, she won't have the more invasive diagnostic testing (CVS or amnio).

What does it mean that this test is "80 percent accurate" in detecting Down syndrome?

You may have read that the results of this test are 80 percent accurate in detecting your risk of having a baby with Down syndrome. That means that if you're carrying a baby with DS, there's an 80 percent chance that the test will pick that up and give you a "screen positive" result indicating that further testing is recommended. It also means there's a 20 percent chance that the test will miss the DS and give you a "screen negative" result, in which case diagnostic testing won't be recommended.

Note: This does not mean that a "screen positive" baby has an 80 percent chance of having DS. It just means that 80 percent of babies who have DS will have screening results that are suspicious enough to recommend diagnostic testing. And 20 percent of babies who have DS will be shown to be at normal risk — that is, the results will be falsely reassuring.

The test also has a 5 percent false positive rate. A "false positive" result is one that suggests there may be a problem when, in fact, there is no problem. In this case, a 5 percent false positive rate means that 5 percent of the babies with normal chromosomes who are tested will be "screen positive" — the test will show them to have an increased risk even though they're normal. Based on this "false positive" result, their mothers may go ahead with invasive diagnostic testing that they otherwise might not have. (And they'll probably worry more, too, until a CVS or amnio shows that their baby is fine.)

What is a first trimester combined screening test?

Some medical centers have added a blood test or two to the nuchal measurement to give you an even more accurate risk assessment. A relatively new screening test called the first trimester combined screening combines the nuchal fold measurement with two blood tests (also done between 11 and 14 weeks), increasing the accuracy of the risk assessment from 80 to 90 percent for Down syndrome. But remember, it's still an estimate of risk and not definitive like CVS or amniocentesis.

The first trimester combined screening isn't available everywhere, but it's becoming more common. The blood tests measure two proteins in your blood: freeBeta-hCG and PAPP-a. A few drops of blood are collected from your fingertips and results are usually available in 24 hours. (Some centers combine the results of some of these tests with the multiple marker screen at 15 to 18 weeks.)

Some medical centers offer different ways of integrating the results of first and second trimester screenings, including the nuchal translucency screening, the blood tests for freeBeta-hCG and PAPP-a, and the multiple marker quadruple screen test. You can either find out the results of each test as you go along or wait until all the tests have been done for a more complete risk analysis. These integrated screenings are 85 to 95 percent accurate in detecting Down syndrome. You can discuss the pros and cons of each option with your caregiver or a genetic counselor.

Finally, a few very advanced medical centers include one more factor in the risk assessment: They look for the baby's nasal bone. If it's not there at all between 11 and 14 weeks, the baby is more likely to have Down syndrome. Including this marker along with the mother's age, baby's age, nuchal fold measurement, and blood tests can bring the accuracy of the risk assessment up to 97 percent.

What are the advantages of the nuchal fold and first trimester combined screening tests?

These screening tests can give you a relatively accurate indication of your baby's risk for chromosomal problems at an early date without subjecting you to the small risk of miscarriage from a more invasive diagnostic test like CVS. If your baby's risk is low, you can find out as soon as possible and may be relieved. If the risk is high you can decide whether to have CVS (done between 11 and 12 weeks), which can tell you for sure whether your baby has a problem while you're still in your first trimester.

The nuchal fold test is not invasive and is no riskier than an ordinary ultrasound. And even if you decide not to have diagnostic testing, you can get more information about your baby's health and development by following up with a routine second trimester ultrasound at 18 to 20 weeks. This ultrasound looks for "soft markers" of chromosome disorders, such as short limbs, a bright dot in the heart, a bright bowel, cysts in a portion of the baby's brain, and certain problems in the kidneys

What's the downside of these screening tests?

Like any screening tests, they're not diagnostic — that is, they can't tell you for sure whether your baby has normal chromosomes. In some cases they cause needless intervention and worry, and in other cases they're falsely reassuring.

Also, the nuchal fold test doesn't detect neural tube defects such as spina bifida and other abnormalities that may be indicated by the multiple marker test (done at 15 to 18 weeks) — but the second trimester ultrasound should be able to detect these problems at least as well as the multiple marker.

What is amniocentesis?

Amniocentesis is a prenatal test that allows you and your healthcare practitioner to gather information about your baby's health and development from a sample of your amniotic fluid. This is the fluid that surrounds your baby in the uterus. An "amnio" is usually done when a woman is between 15 and 20 weeks pregnant to determine whether her baby has genetic or chromosomal abnormalities, such as Down syndrome. But not all moms-to-be choose to have the test, in part because there's a small risk of miscarriage.

Other reasons that you might have amniocentesis include:

• To determine whether your baby's lungs are mature enough for an early delivery if you appear to be in premature labour or require an early delivery for any reason.

• To diagnose or rule out a uterine infection if your water has broken prematurely or your practitioner has any other reason for suspecting you have one.

• To check on the well-being of your baby if you have a blood sensitization, such as Rh sensitization. This is a complex condition that can occur if your blood is a different type than your baby's. (Note: More and more hospitals are using Doppler ultrasound.  for this purpose instead of amnio.)

Which birth defects can the test detect?

Amniocentesis can identify several hundred genetic disorders, including some of the most common:

• Nearly all chromosomal disorders, including Down syndrome and Edward's syndrome. The test is more than 99 percent accurate in diagnosing these conditions. An ultrasound, a combined first-trimester screening, or a multiple marker test may indicate whether your baby is likely to have these problems, but only amniocentesis can tell you for sure.

• Other genetic disorders such as cystic fibrosis, sickle cell disease, Tay-Sachs disease, and Huntington's disease. Neither the multiple marker test nor ultrasound will pick up these problems. Only amniocentesis can provide the information needed to diagnose them in the womb.

• Amniocentesis can also help detect neural tube defects such as spina bifida and anencephaly.

More than 95 percent of the high-risk women who have amniocentesis receive good news from the results. Only about 5 percent will be found to have a baby with a problem. Amniocentesis doesn't detect every birth defect, however. A cleft lip or palate, for example, won't show up on the test.

Is this test appropriate for me?

Although most testing centers will do an amnio for any pregnant woman who wants one, the procedure isn't offered routinely to all pregnant women because there's a small risk of miscarriage. It's usually only offered to those found to have a somewhat increased risk of having a baby with a chromosomal or genetic defect. You may be in this category if:

• You'll be 35 years old or older on your due date. The risk of having a child with a chromosomal defect rises as a woman ages. The chance that you're carrying a baby with Down syndrome, for instance, is about 1 in 300 when you're 35 years old — compared with 1 in 1,150 when you're 25.

• You've had another test, such as a multiple marker screen, a nuchal fold scan, or a "combined screening," that indicated your baby is at a higher risk for a problem. If this is the case, you might first have a detailed ultrasound to look for physical signs of Down syndrome and other defects. If the ultrasound shows that there's still cause for concern, you may want to have amniocentesis for a definitive diagnosis.

• You've previously been pregnant with a child with a birth defect.

• You or your partner has a chromosomal abnormality or genetic disorder or a family history that makes it more likely that your child will have genetic problems. Or you and your spouse are both carriers of a recessive genetic disorder such as cystic fibrosis or sickle cell disease.

What are the risks of the procedure?

In deciding whether to have this test, you'll need to weigh your desire to know about your baby's condition against the slight risk that the test will lead to a miscarriage. You'll definitely want to discuss your options with a genetic counselor so that you understand all the risks and benefits of your choices. For example, you may decide to have a combined first-trimester screening or a multiple marker test first to gain more information about your risk for certain problems.

There are many reasons to want to know about your child's condition. Even if you would never terminate a pregnancy for any reason, knowing in advance that your baby will have special needs allows you to prepare for the challenges you might face. You might want to switch to a better-equipped hospital with specialists, for example. Knowing what's going on with your baby allows your medical team to monitor your pregnancy as needed and to bring a neonatologist or pediatric surgeon on board to prepare to help your baby after delivery.

A few problems can even be treated in utero, such as biotin dependence and MMA (methylmalonic acidemia) — both life-threatening, though extremely rare, body chemistry disorders. More often, when a female baby is suspected to be at risk for congenital adrenal hyperplasia, giving the mother hydrocortisone can prevent the baby from developing the masculine features associated with this condition.

That said, you'll want to consider the small risk that the procedure will cause you to miscarry. According to the Centers for Disease Control and Prevention (CDC), the risk of miscarriage from amniocentesis is between one in 200 and one in 400, depending on the skill and experience of the doctor performing it. You'd also have a slight risk of uterine infection in the days following the procedure (less than one in 1,000), which can sometimes lead to miscarriage. These are all things you should discuss with your partner, your practitioner, and a genetic counselor.

Is there any way to reduce the risks?

The more times the doctor who does the amnio has performed the procedure, the lower your risk of complications. Ask your practitioner or genetic counselor to refer you to a doctor with plenty of experience, preferably one who does at least 50 amnios a year.

You'll also want to make sure that an experienced registered diagnostic medical sonographer provides continuous ultrasound guidance during the procedure. This greatly improves the chances that the doctor will be able to obtain enough fluid on the first try and avoid having to repeat the procedure. And when continuous ultrasound guidance is used, injuries to the baby from the amnio needle are very rare.

Can I meet with a counselor before deciding whether to have the test?

Most testing centers require that you meet with a genetic counselor to discuss the risks and benefits of various methods of prenatal testing before you have the test. The counselor will take down your family history and ask questions about your pregnancy.

Your answers will enable the counselor to give you a sense of your risk for a problem and determine whether you need to be screened for a particular genetic disease. Then you can decide whether you want to go ahead with the amnio.

What's the procedure like?

Before you have your amnio, an ultrasound is done to measure your baby and check his basic anatomy. This may happen on the same day as the amnio or a few days or weeks beforehand.

For the amnio itself, you lie on an examining table and your belly is cleaned with alcohol or an iodine solution. (Someone may stay with you throughout the procedure.) A doctor or technician uses ultrasound to pinpoint a pocket of amniotic fluid a safe distance from both the baby and the placenta. This part can take up to 20 minutes.

Then, under continuous ultrasound guidance, the doctor inserts a long, thin, hollow needle through your abdominal wall and into the sac of fluid around your baby. She withdraws a small amount of amniotic fluid — about an ounce, or two tablespoons — and then removes the needle. Withdrawing the fluid should take less than 30 seconds. And your baby will make more fluid to replace what's taken out.

You may feel some cramping, pinching, or pressure during the procedure — or you may feel no discomfort at all. The amount of discomfort or pain varies among women and even from one pregnancy to the next. You may choose to have your abdomen numbed first with a local anesthetic, but the pain from the anesthesia injection is likely to be worse than that of the amnio itself and most moms-to-be decide that one needle is enough.

If you're worried about a needle being that close to your baby, be assured that direct injury to the baby from amniocentesis is very rare with continuous ultrasound guidance. Your doctor will avoid placing the needle near the baby, but if your child happens to come in contact with it, he'll move away quickly just as you would if you bumped into something sharp.

Afterward, your doctor may use an external fetal monitor to listen to the baby's heartbeat for reassurance. You'll want to take it easy for the rest of the day and avoid any heavy lifting for the next two days. You may have some minor cramping for a day or so. About 1 to 2 percent of women will have significant cramping, vaginal spotting, or leaking amniotic fluid. Call your practitioner if you have any of these symptoms — or a fever — because they could be signs of impending miscarriage.

Note: If your blood is Rh-negative, you'll need a shot of Rh immunoglobulin after amniocentesis (unless the baby's father is Rh-negative as well) because it's possible that your baby's blood mixed with yours during the procedure.

When will I get the results?

You should have the full results within two weeks, although you can sometimes get results in as few as eight days. During this waiting period, a laboratory analyzes the fluid sample, first measuring the amount of alpha-fetoprotein (AFP) in the fluid. This shows whether there's likely to be an opening in the baby's skin, indicating a neural tube defect such as spina bifida or anencephaly. The lab also takes some of the baby's living cells from the fluid and allows them to reproduce for a week or two, then tests the cells for chromosomal abnormalities and evidence of certain genetic birth defects.

How do I decide whether to have an amnio or the CVS test?

Both these tests can tell you whether your baby has a chromosomal problem. The CVS test is done earlier in pregnancy (usually between 11 and 12 weeks), so you can find out sooner about your baby's condition. Some centers offer amniocentesis during the first trimester, but research indicates that this practice leads to a higher rate of miscarriage and it's considered experimental. Here are some considerations:

The CVS may be a better choice if you want to know your baby's condition while you're still in your first trimester. You may have heard that the risk of miscarriage from the CVS is slightly higher than from amniocentesis, but the most recent research has found this to be untrue.

Amniocentesis may be a better choice if you decide you want to wait for the results of a multiple marker screening (usually done between 15 and 20 weeks) before subjecting yourself to a more invasive test. (The CVS must be done by 12 weeks to be effective.)