Prenatal Tests @ Which Trimester

Prenatal tests give you information about your health and your baby’s, when you’re pregnant. They help detect any problems that could affect him, like birth defects or genetic diseases. The results can help you make the best health care decisions before and after your child is born.

Prenatal tests are helpful, but it’s more important to interpret what they find. A positive test result doesn’t always mean your baby will be born with a disorder. You must talk with your doctor, midwife, or other health care provider about what the tests mean and what you should do once you have the results.

Doctors recommend some prenatal tests for all pregnant women. Only some women will need other screening tests to check for certain genetic problems.

The following are some of the more common trimester wise tests performed during pregnancy:

First Trimester Prenatal Screening Tests

First trimester screening is a combination of fetal ultrasound and maternal blood testing performed during the first trimester of pregnancy. This screening process can help to determine the risk of the fetus having certain birth defects. Screening tests may be used alone or in combination with other tests.

There are three parts of first trimester screening:

  1. Ultrasound test for fetal nuchal translucency (NT).

  2. Two maternal serum (blood) tests. The blood tests measure two substances found in the blood of all pregnant women: 
    -   Pregnancy-associated plasma protein screening (PAPP-A)--a protein produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk for chromosome abnormality.
    -   Human chorionic gonadotropin (hCG)--a hormone produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk for chromosome abnormality.

When used together as first trimester screening tests, nuchal translucency screening and maternal blood tests have a greater ability to determine if the fetus might have a birth defect, such as Down syndrome (trisomy 21) and trisomy 18.

If the results of these first trimester screening tests are abnormal, genetic counseling is recommended. Additional testing such as chorionic villus sampling, amniocentesis, cell-free fetal DNA, or other ultrasounds may be needed for accurate diagnosis.

Second Trimester Prenatal Screening Tests

It's the second trimester, and you've known you are pregnant for some time now. But what can you expect yourself? What tests will you get during the fourth, fifth and sixth months of pregnancy? At an early prenatal visit, you may be asked if you would like genetic testing. Or, if you are aware of a genetic condition in your family or your partner's, you or your partner may opt to get tested before conceiving. Also, your health care professional will talk with you about a variety of prenatal tests to assess the health of the fetus. It's up to you which ones you have done or wants to do. For instance, if you have no intention of terminating the pregnancy if the tests do find a problem, you may want to skip them. However, even then, you may want to have the test so you can prepare yourself emotionally for the possibility of having a special-needs child.

While the tests may get a bit more complicated, it is only because your baby is getting more complex. He or she is growing steadily, forming features that are some combination of you and your mate. While you may have gotten an initial ultrasound during the first trimester to confirm your baby's basic health, it is during the second trimester that an ultrasound may confirm whether your baby is a boy or a girl. Also, this ultrasound will probably be the first in which you can see your baby's features in profile.

Remember, although there is a slight risk of complications with some of these tests, they are routinely performed to help ensure good health for you and your baby. Talk to your health care provider to learn more details about the risks and benefits of these prenatal exams.

There are two types of prenatal tests:

  1. Screening tests and

  2. Diagnostic tests.

Prenatal screening is meant to identify women at an increased risk for the pregnancy to be affected by a disease and Diagnostic prenatal testing is used to confirm whether or not a pregnancy is affected by a given condition.

Serum screening test: This is performed with a blood sample and is usually done between weeks 15 and 18 of the pregnancy (as measured from the start of the most recent menstrual period). The test estimates your risk of having a baby with:

  • An open neural tube defect. When the neural tube fails to close, the baby is born with an opening in the head (anencephaly) or spinal cord (spina bifida). Babies with anencephaly are stillborn or die soon after birth; those with spina bifida need surgery and may be paralyzed. The severity of spina bifida can vary greatly. One way to reduce your risk of having a child with a neural tube defect is to take folic acid (400 micrograms per day) before conception.
  • Down syndrome (also called trisomy 21). An extra copy of chromosome 21 causes Down syndrome characterized by mental retardation, certain facial features and sometimes heart defects.
  • Trisomy 18. An extra copy of chromosome 18 causes this syndrome, which usually proves fatal during the first year of the baby's life and is associated with severe mental retardation.

The blood screen also known as triple test does not look directly at genetic material but instead measures four substances—alpha-fetoprotein, unconjugated estriol and human chorionic gonadotropin and Inhibin A—to determine whether you are at increased risk of having a baby with one of these disorders. During the second trimester, you will probably be encouraged to take a so-called triple test, a blood test that checks these three hormone and the protein levels (Inhibin A) in your blood.

An important part of the triple test is the alpha-fetoprotein screening (MSAFP) test. The MSAFP, which is usually performed between weeks 15 and 20, checks the level of alpha-fetoprotein in your bloodstream. Alpha-fetoprotein is a key indicator of fetal health. All women are typically given this test; it is not just for at-risk pregnancies. If your triple test results come back abnormal, you will be offered an amniocentesis to provide more definitive results. Both tests are typically taken between the 15th and 20th weeks of pregnancy.

A key fact to remember is that this test does not diagnose the disorders—it only screens for the substances and shows if there is an increased risk of a disorder. Further testing is always suggested to make a diagnosis.

Nuchal translucency screening test: A newer type of prenatal screening test that can be done in the first trimester is becoming much more common, although it's still not offered everywhere. This test assesses the risk for the fetus to have Down syndrome or trisomy 18. The first trimester screening has two parts and both should both be performed between weeks nine and 13 of gestation. One part tests levels of maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) in the mother's blood. The other measures the nuchal thickening (measure of fetal neck thickness) through a specialized ultrasound. This test detects 85 percent of fetuses with Down syndrome and approximately 90 percent with trisomy 18.

However, remember that it is a screening test and still requires a confirmatory chorionic villous sampling (CVS) or amniocentesis to make the definite diagnosis. However, the primary benefit of this screening test is that it can be performed earlier in the pregnancy and is much less invasive than a CVS or amniocentesis.

Next steps after receiving abnormal screening results

A screening test does not diagnose a condition. It should not be used to make decisions for treatment or pregnancy termination, but rather should be used to determine whether the mother wishes to have diagnostic prenatal testing performed.

If a screening test indicates a higher-than-average risk, your health care professional may want to perform a basic ultrasound, which can help determine the gestational age of the fetus and show if a woman is carrying twins. If either of these factors accounts for the abnormal screen test result, no further testing is needed. Otherwise, a more detailed ultrasound examination may be performed, which allows a look at the baby's brain and spinal cord, as well as other parts of the body. This ultrasound can often identify an open neural tube defect or other malformation associated with an abnormal screening test.

Your health care provider may suggest you consider either amniocentesis or CVS. Both are diagnostic tests to determine whether the fetus actually has the disorder in question.

Amniocentesis is performed more frequently and should be the choice if you're at risk for having a child with neural tube defects (CVS doesn't detect neural tube defects).

Amniocentesis

It is not recommended for all women—mainly due to the fact that the test is invasive and carries a small risk of miscarriage—but it is generally administered to those who are at increased risk for genetic and chromosomal problems. This prenatal screening enables your health care professional to examine fetal cells in the amniotic fluid for any chromosomal abnormalities.

If you are 35 or older when you're due to have your baby, your health care professional will likely discuss the risks of chromosomal abnormalities based on your age and recommend this test. That's because women over 35 have a higher risk of having a baby with Down's syndrome.

An amniocentesis is also recommended if you've already had a child with certain birth defects, or if you have a family or personal history that puts you at risk for certain inherited diseases. You might choose to have this test if you had abnormal blood tests that suggest there might be a problem.

Amniocentesis can diagnose numerous conditions, but as these tests are very expensive, so talk with your health care professional about which ones are necessary based on your history and risk factors.

During an amniocentesis, the doctor inserts a needle through your abdomen into the amniotic sac and removes a small amount of amniotic fluid. The doctor uses ultrasound to guide the needle and avoid inserting it into the placenta. The test can be performed on an outpatient basis in a health care professional's office or in a hospital.

It can be done at any gestational age after 11 weeks, but when it's performed for genetic studies, amniocentesis is usually done between 15 and 17 weeks

If you have amniocentesis, a doctor will use a needle to withdraw a sample of amniotic fluid (the fluid surrounding the fetus) for analysis. The procedure is typically performed during the second trimester, at 15 to 18 weeks.

Amniocentesis or CVS is offered for high-risk pregnancies, which could include any of the following:

  • you will be 35 years of age or older at delivery

  • your family has a known genetic disorder

  • you have had a previous child with a birth defect

  • you and your partner are carriers of the same recessive disorder

  • other indications as directed by your obstetrician

CVS: It is performed at 10 to 12 weeks of gestation and involves analyzing a sample of placental tissue. A thin tube inserted through the vagina and cervix, or the abdomen, and is used to suction out a small tissue sample from the placenta. However, unlike amniocentesis, CVS cannot be used to test for neural tube defects, such as spina bifida and anencephaly. Therefore, it's usually recommended that a woman undergoing CVS also have the prenatal blood test called the maternal serum alpha fetoprotein (MSAFP) screening test, at about 16 to18 weeks of pregnancy. 

CVS is more attractive to some women because it can be done earlier than amniocentesis. Amniocentesis and CVS can be used to prenatally diagnose many, but not all, genetic disorders. 

“Every prenatal screening and diagnostic test has its benefits, risks and limitations. It is very important to discuss these testing options with your doctor and, if possible, a genetic counselor.” 

Third Trimester Prenatal Screening Tests

The third trimester will be an especially busy time for you and your baby. You'll be glowing, putting the last touches on the baby's room and checking with your nurse or doctor almost weekly to keep your baby healthy and kicking.

As far as checkups, exams and medical tests go, the third trimester is different from the first two. Your baby is quite developed. It's beginning to look like it will at delivery. At this stage, it is less sensitive to birth defect-causing chemicals than it was during the first six months. Its size also means it may be more easily checked for physical defects that were previously too small to see.

Since your baby is still developing and is fitting tighter and tighter within the womb environment, it's good to continue having plenty of visits to your doctor. Your health care provider will set up a visit schedule for you, but you can probably expect weekly checkups at this point.

Included in these visits will be an increasing number of pelvic exams.

Your health care provider will be checking for just two things during a pelvic examination: 

  • The baby's positioning and

  • The state of your cervix. 

During the last three months of pregnancy, most babies invert, with their feet pointed at—and kicking endlessly at—your diaphragm and their heads pointed downward toward your cervix. If your baby is not inverted, your health care provider may talk to you about methods for getting the baby to turn, with the last option being planning a Caesarean section for you.

Likewise, your cervix will slowly begin to dilate and get thinner during the last month or so of your pregnancy to make way for delivery. Your OB/GYN or midwife will keep track of your cervical thinning, which is called effacement, and your dilation.

One test you should expect during the third trimester is a group B streptococcus (GBS) test. GBS is a common bacterium that is mostly harmless in children and adults. Its cousin, group A streptococcus, is the one that causes strep throat. In developing fetuses, though, GBS can cause a serious infection. Testing for GBS is relatively simple and involves little more than a vaginal swab to check for the presence of the bacteria. 

If you go past your due date, you can expect to take a fetal non-stress test (NST), which involves little more than donning two belts during a checkup, one to measure your baby's heart rate and the other your contraction rate.

Your provider may order two other tests to ensure that your child isn't experiencing physical problems before birth. One is a biophysical profile, which combines an NST and an ultrasound. It is used to make sure the baby's heart rate, movement, muscle tone and breathing are normal. The second exam is a glucose challenge screening, which tests whether you have developed gestational diabetes.

The ideal time for many tests is of course before you get pregnant. If you are planning a pregnancy, preconception genetic counseling can be quite helpful because it offers an opportunity to make decisions without the pressure of an advancing pregnancy. Potential risks for the planned fetus can be identified and testing strategies can be pursued.

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