Genetic Screening
Many genetic abnormalities can be diagnosed before birth. Your doctor or midwife may recommend genetic testing during pregnancy if you or your partner has a family history of genetic disorders. You may also choose to have genetic screening if you have had a fetus or baby with a genetic abnormality.
Examples of genetic disorders that can be diagnosed before birth include
Cystic fibrosis
Duchenne muscular dystrophy
Hemophilia A
Polycystic kidney disease
Sickle cell disease
Tay-Sachs disease
Thalassemia
The following screening methods are available during pregnancy
Alpha-fetoprotein (AFP) test or multiple marker test
Amniocentesis
Chorionic villus sampling
Cell-free fetal DNA testing
Percutaneous umbilical blood sampling (withdrawing a small sample of the fetal blood from the umbilical cord)
Ultrasound scan
First Trimester Prenatal Screening Tests
First trimester screening is a combination of fetal ultrasound and maternal blood testing. This screening process can help determine the risk of the fetus having certain birth defects. Screening tests may be used alone or with other tests.
First trimester screening includes
Ultrasound for fetal nuchal translucency. Nuchal translucency screening uses an ultrasound to examine the area at the back of the fetal neck for increased fluid or thickening.
Ultrasound for fetal nasal bone determination. The nasal bone may not be visualized in some babies with certain chromosome abnormalities, such as Down syndrome. This screen is performed using an ultrasound between 11 and 13 weeks gestation.
Maternal serum (blood) tests. These blood tests measure two substances found in the blood of all pregnant women:Pregnancy-associated plasma protein A. A protein produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk of chromosomal abnormality.Human chorionic gonadotropin. A hormone produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk of chromosomal 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 an accurate diagnosis.
Second Trimester Prenatal Screening Tests
Second trimester prenatal screening may include several blood tests called multiple markers. These markers provide information about your potential risk of having a baby with certain genetic conditions or birth defects. Screening is usually done by taking a sample of your blood between 15 and 20 weeks of pregnancy (16 to 18 weeks is ideal). The multiple markers include:
AFP screening. Also called maternal serum AFP, this blood test measures the level of AFP in your blood during pregnancy. AFP is a protein normally produced by the fetal liver that is present in the fluid surrounding the fetus (amniotic fluid). It crosses the placenta and enters your blood. Abnormal levels of AFP may indicate:A miscalculated due date, as the levels vary throughout pregnancyDefects in the abdominal wall of the fetusDown syndrome or other chromosomal abnormalitiesOpen neural tube defects, such as spina bifidaTwins (more than one fetus is producing the protein)
Estriol. This is a hormone produced by the placenta. It can be measured in maternal blood or urine to be used to determine fetal health.
Inhibin. This is a hormone produced by the placenta.
Human chorionic gonadotropin. This is also a hormone produced by the placenta.
Abnormal test results of AFP and other markers may mean that additional testing is needed. An ultrasound is used to confirm the milestones of your pregnancy and to check the fetal spine and other body parts for defects. An amniocentesis may be needed for an accurate diagnosis.
Since multiple marker screening is not diagnostic, it is not 100 percent accurate. It helps determine who in the population should be offered additional testing during pregnancy. False-positive results may indicate a problem when the fetus is actually healthy. On the other hand, false-negative results indicate a normal result when the fetus actually does have a health problem.
When you have both first and second trimester screening tests performed, the ability of the tests to detect an abnormality is greater than using just one screening independently. Most cases of Down syndrome can be detected when both first and second trimester screenings are used.
Ultrasound
An ultrasound scan is a diagnostic technique that uses high-frequency sound waves to an image of the internal organs. A screening ultrasound is sometimes done during the course of your pregnancy to check normal fetal growth and verify the due date.
When are ultrasounds performed during pregnancy?
Ultrasounds may be done at various times throughout pregnancy for several reasons
First Trimester
To establish the due date (this is the most accurate way of determining the due date)
To determine the number of fetuses and identify placental structures
To diagnose an ectopic pregnancy or miscarriage
To examine the uterus and other pelvic anatomy
To detect fetal abnormalities (in some cases)
Midtrimester (also called the 18- to 20-week scan)
To confirm the due date (a due date set in the first trimester is rarely changed)
To determine the number of fetuses and examine the placental structures
To assist in prenatal tests, such as an amniocentesis
To examine the fetal anatomy for abnormalities
To check the amount of amniotic fluid
To examine blood flow patterns
To observe fetal behavior and activity
To measure the length of the cervix
To monitor fetal growth
Third Trimester
To monitor fetal growth
To check the amount of amniotic fluid
To conduct the biophysical profile test
To determine the position of the fetus
To assess the placenta
How is an ultrasound scan performed?
Two types of ultrasounds can be performed during pregnancy
Abdominal ultrasound. In an abdominal ultrasound, gel is applied to your abdomen. The ultrasound transducer glides over the gel on the abdomen to the image.
Transvaginal ultrasound. In a transvaginal ultrasound, a smaller ultrasound transducer is ed into your vagina and rests against the back of the vagina to an image. A transvaginal ultrasound produces a sharper image than an abdominal ultrasound and is often used in early pregnancy.
Which ultrasound imaging techniques are available?
There are several types of ultrasound imaging techniques. As the most common type, the 2-D ultrasound provides a flat picture of one aspect of the baby.
If more information is needed, a 3-D ultrasound exam can be done. This technique, which provides a 3-D picture, requires a special machine and special training. The 3-D image allows the health care provider to see the width, height and depth of the images, which can be helpful during the diagnosis. The 3-D images can also be captured and saved for later review.
The latest technology is 4-D ultrasound, which allows the health care provider to visualize the unborn baby moving in real time. With 4-D imaging, a three-dimensional image is continuously d, providing a “live action” view. These images often have a golden color, which helps show shadows and highlights.
Ultrasound images may be captured in still photographs or on video to document findings.
What are the risks and benefits of ultrasound imaging?
Fetal ultrasound has no known risks other than mild discomfort due to pressure from the transducer on your abdomen or in your vagina. No radiation is used during the procedure.
Transvaginal ultrasound requires covering the ultrasound transducer in a plastic or latex sheath, which may cause a reaction in women with a latex allergy.
Ultrasound imaging is constantly being improved and refined. As with any test, the results may not be completely accurate. However, an ultrasound can provide valuable information to parents and health care providers, helping them manage and care for the pregnancy and the baby. In addition, ultrasound imaging gives parents a unique opportunity to see their baby before birth, helping them to bond and establish an early relationship.
Fetal ultrasound is sometimes offered in nonmedical settings to provide keepsake images or videos for parents. While the ultrasound procedure itself is considered safe, it is possible that untrained personnel may miss an abnormality or give parents false assurances about their baby’s well-being. It is best to have an ultrasound performed by trained medical personnel who can correctly interpret the results. Talk with your doctor or midwife if you have questions.
Amniocentesis
An amniocentesis involves taking a small sample of the amniotic fluid that surrounds the fetus. It is used to diagnose chromosomal disorders and open neural tube defects, such as spina bifida. Testing is available for other genetic defects and disorders depending on your family history and the availability of lab testing at the time of the procedure.
Who is an ideal candidate for amniocentesis?
An amniocentesis is generally offered to women between the 15th and 20th week of pregnancy who have an increased risk of chromosomal abnormalities. Candidates include women who will be over age 35 at the time of delivery or those who have had an abnormal maternal serum screening test.
How is an amniocentesis performed?
An amniocentesis involves ing a long, thin needle through your abdomen into the amniotic sac to withdraw a small sample of amniotic fluid. The amniotic fluid contains cells shed by the fetus, which contain genetic information. Although specific details of each procedure may vary, a typical amniocentesis follows this process:
Your abdomen will be cleansed with an antiseptic.
Your doctor may or may not give a local anesthetic to numb the skin.
Your doctor will use ultrasound technology to help guide a hollow needle into the amniotic sac. He or she will withdraw a small sample of fluid for lab analysis.
You may feel some cramping during or after the amniocentesis. Strenuous activities should be avoided for 24 hours following the procedure.
Women who are pregnant with twins or other higher-order multiples need sampling from each amniotic sac to study each baby. Depending on the position of the baby and placenta, amount of fluid, and woman’s anatomy, sometimes the amniocentesis cannot be done. The fluid is then sent to a genetics lab so that the cells can grow and be analyzed. AFP is also measured to rule out an open neural tube defect. Results are usually available in about 10 days to two weeks, depending on the lab.
Chorionic Villus Sampling (CVS)?
CVS is a prenatal test that involves taking a sample of some of the placental tissue. This tissue contains the same genetic material as the fetus and can be tested for chromosomal abnormalities and some other genetic problems. Testing is available for other genetic defects and disorders, depending on your family history and the availability of lab testing at the time of the procedure. Unlike amniocentesis, CVS does not provide information on open neural tube defects. Therefore, women who undergo CVS also need a follow-up blood test between 16 and 18 weeks of pregnancy to screen for these defects.
How is CVS performed?
CVS may be offered to women with an increased risk of chromosomal abnormalities or who have a family history of a genetic defect that is testable from the placental tissue. CVS is usually performed between the 10th and 13th week of pregnancy. Although exact methods may vary, the procedure involves the following steps:
Your doctor will a small tu
