Special Tests During Pregnancy

During your prenatal care you can have a number of special screening, and diagnostic tests to check for various complications or defects affecting the baby. These tests can be reassuring if they rule out something you're worrying about, such as a genetic defect like cystic fibrosis, but they may also reveal a problem that makes you question whether you want to go on with your pregnancy. Situations like this put an enormous emotional strain for parents-to-be, so it's important to talk in detail to your doctor beforehand about the risks of having the tests and the implications of their results.

Screening Tests

Most maternity dinics now have a number of tests available to screen mothers for a variety of possible fetal abnormalities. These screening tests don't tell you for certain whether anything is wrong, but if a test shows that there may be a problem, you may have some more diagnostic tests to confirm or rule it out.

Nuchal Translucency (NT) Scan The risk of having a baby with Down syndrome or some other chromosomal defect can be assessed around 11-14 weeks using a special ultrasound scan called a nuchal translucency scan. The ultrasound measures the thickness of the skin at the back of the fetus's neck-it's thicker in babies with Down syndrome. On the same day as the scan, you'll have a finger prick blood test for levels of pregnancy­associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG).

Combining a blood test with the nuchal scan raises the sensitivity of the screen to about 91 percent. Based on the results of blood tests, nuchal translucency measurement, and your age, a physician is able calculate your baby's risk of Down syndrome. A positive test does not mean that your baby has Down syndrome, however-only that your risk is greater.

Triple serum screen This test was developed by St. Bartholomew's Hospital in London. A sample of the pregnant mother's blood is taken between 14 and 20 weeks to measure the levels of three substances-estriol, human chorionic gonadotrophin, and alpha­fetoprotein. The results are assessed in relation to the mother's age to predict the chance of her baby's suffering from Down syndrome. If the chances seem high (more than one in 250), doctors will suggest you have amniocentesis. If you're not offered a triple serum screen, you can ask to have it.

AFP test Alpha-fetoprotein is found in varying amounts in your blood throughout pregnancy. Between 16 and 18 weeks the levels are usually low. If a blood test is done at this time and the levels are 2-3 times higher than the average of a sample group, it may show that there's a neurological problem. Some neural tube defects are too small to detect with ultrasound alone. If the AFP test is abnormal, you'll have a "targeted" scan of the spine, abdomen, and placenta; and perhaps an amniocentesis. Although nuchal translucency measurement has replaced AFP testing in many dinics as a screening test for Down syndrome (see opposite), it cannot replace screening for neural tube defects.

Diagnostic Tests

These tests are used to confirm abnormalities in the fetus, and are generally only used after screening tests or ultrasound scans have shown that you may be at a high risk. The main diagnostic tests are amniocentesis and chorionic villus sampling (CVS). Amniocentesis is the most common diagnostic test; CVS is not available in all dinics, and it carries a higher risk of miscarriage, but it does give an early diagnosis.

You and your partner will need to think very carefully about the implications of having these tests and talk them through together and with your doctor. Ask for special counseling if you think you may need it.

Amniocentesis

Amniotic fluid contains cells from the baby's skin and other organs that can be used to diagnose his condition. Amniocentesis is the name given to the procedure that withdraws this fluid from the uterus.

Why it's done You'll probably want to have amniocentesis if you are over the age of 35, as the risk of chromosomal abnormalities (such as Down syndrome) increases with age (see column, right). It may also be suggested after serum screening, or if a nuchal scan shows a risk of Down syndrome (see opposite). Amniocentesis can also reveal other important information that may be sometimes helpful in determining the care and progress of your pregnancy.

Amniocentesis can be used to check for metabolic disorders, but it is preferable to have them diagnosed earlier in pregnancy by CVS. At one time amniocentesis was also used to check the bilirubin content of the fluid to help work out if a Rhesus-positive baby needed a blood transfusion while still in the mother's uterus, but this is now done by a Doppler scan to check for fetal anemia. What it can reveal Where there is cause for concern, an amniocentesis test may show the following:

• The sex of the baby: cells sloughed off by the fetus accumulate in the amniotic fluid. Under the microscope, male cells can be distinguished from female cells and the baby's sex determined. In gender-linked genetically linked disorders such as hemophilia, a male child will have a 50 percent chance of being affected.
• The chemical composition of the fluid: this can reveal metabolic disorders caused by missing or defective enzymes
• The chromosome count: determined by examining discarded cells. Any deviation from the normal chromosomal structure usually means that the baby may have a disability.

How is it done? Amniocentesis is usually carried out at 16-18 weeks. Guided by ultrasound, the doctor inserts a hollow needle into the amniotic sac through the front of the abdominal wall. About 20cc (a few tablespoons) of amniotic fluid is usually with­drawn, and this is then spun in a centrifuge to separate the cells shed by the baby from the rest of the liquid. The cells then have to be cultured, and it takes about three weeks for the results to come through-a very stressful period for couples. Many women talk about putting their pregnancies "on hold" during this time, until the results confirm that the baby is unaffected.

Amniocentesis is only undertaken with ultrasound monitoring to guide the needle into the amniotic sac, so that neither the placenta nor the fetus is harmed. The risk that the procedure will induce a miscarriage in early pregnancy is small-about one in 200. There's also a very small risk (less than one percent) of your waters breaking, and your baby could develop respiratory difficulties after birth.

Chorionic Villus Sampling (CVS)

Chorionic villi, fingerlike outgrowths on the edge of the chorion, are genetically identical to the fetus. They develop earlier than amniotic fluid, so examining a sample of chorionic villi provides valuable information about your baby's genes and chromosomes before it's possible to carry out amniocentesis between 10-11 weeks.

What it can reveal The most important group of mothers needing CVS are those at risk of having a Down syndrome baby. An abnormality of hemoglobin, such as sickle-cell disease or thalassemia, can also be diagnosed with CVS. Inborn errors of metabolism are, fortunately, rare, but if a family is afflicted, the incidence may be as high as one in four. The basic defect is an enzyme deficiency, and direct enzyme analysis on the chorionic tissue gives a diagnosis within two days. Single gene disorders, such as cystic fibrosis, hemophilia, Huntington's chorea, and muscular dystrophy, can be detected with the use of CVS.

How is it done? CVS is also carried out under ultrasound control, usually between 10 and 12 weeks, before the amniotic sac completely fills the uterine cavity. Two routes are used, the transcervical route and the transabdominal route. For the first, the cervix is first examined using a speculum. A plastic or metal catheter is then introduced through the cervical canal, across the uterine cavity, and then into the outside edge of the placenta. A small amount of chorionic villi tissue is then removed for analysis.

The second procedure follows that of amniocentesis, but with a sample being taken of the placental tissue rather than of the amniotic fluid. The risk of miscarriage following CVS is about one percent higher than the spontaneous miscarriage rate. The advantage. of CVS is that it gives an initial result within 24-48 hours, with full results in about a week. This is helpful if the risks are high and you don't want to have to wait until your pregnancy is at a more advanced stage for the results of amniocentesis.

Umbilical Veib Sampling (Corodocentesis)

Also known as Percutaneous Umbilical Cord Blood Sampling (PUBS), this procedure is used to check the makeup of fetal blood and, in cases of fetal anemia, for intrauterine blood transfusion. It's also used to check for infections and to assess your baby's levels of hemoglobin. Umbilical vein sampling is no longer used in cases of suspected slow growth. Instead, Doppler scans provide the necessary information and these are available in most clinics.

Infection detection Rubella, toxoplasmosis, and the herpes virus may be detected by performing a specific radio analysis of certain proteins that are present in a baby's blood.

Rhesus isoimmunization In cases of Rhesus incompatibility, assessing the baby's hemoglobin is the best way to determine the severity of blood-cell destruction and whether a blood transfusion for the baby in the womb (also done through the umbilical vein) needs to be carried out.

How is it done? Under ultrasonic control, a hollow needle is passed through the front wall of the mother's abdomen and uterus into a blood vessel in the umbilical cord, about half an inch (1 cm) from where it emerges from the placenta. A small quantity of blood can then be removed for testing: The risk to the fetus appears to be about 1-2 percent. Umbilical vein sampling can replace any investigation currently undertaken on a blood sample.

Doppler Scan

The Doppler scan uses black-and-white or color images to look at the blood flow between the placenta and your baby through the Umbilical cord. It uses a slightly different type of sound wave from a normal ultrasound scan, which bounces off moving red blood cells and shows how fast they're moving through the fetus's blood vessels. Doppler is used to check when a baby is small for dates or seems not to be growing as fast as it should.

Doppler can be used to assess whether the developing fetus has anemia, although the only way to tell for certain is by the PUBS test. Doppler is also used to find out whether a Rhesus-positive baby needs a blood transfusion in the womb.

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