Genetic screening is used to determine whether a couple is at increased risk of having a baby with a hereditary genetic disorder. To determine whether having a baby with a genetic disorder is likely, doctors ask the couple about disorders that family members have had and about the cause of death in family members. Information about three generations is usually needed. Doctors also ask about the health of all living first-degree relatives (parents, siblings, and children) and second-degree relatives (aunts, uncles, and grandparents). Information about miscarriages, stillborn babies, or babies who have died soon after birth is also helpful, as is information about intermarriages among relatives and ethnic background.

How Is First Trimester Screening Performed?

Strict quality controlled certification is required to perform this screening. Dr. Kamran Torbati is among handful number of physicians in San Fernando Valley that is certified to perform the Nuchal Tranlucency ultrasound at his office by the Fetal Medicine Foundation.  A finger-stick blood sample for the biochemical assays is performed in conjunction with the genetic counseling and ultrasound. The blood sample is analyzed by NTD Labs, Inc.

Who Is Eligible For First Trimester Screening?

First trimester screening is designed to detect chromosomal abnormalities in low-risk pregnancies in younger mothers. Women older than 34 as well as couples with additional risk factors may also consider other diagnostic testing procedures, such as Amniocentesis or Chorionic villus sampling (CVS).
 

Second Trimester maternal serum screening for fetal open neural tube defects and aneuploidy

Maternal serum screening has been modified during the past 25 years and is now widely utilized during the second trimester to identify women at risk for fetal open neural tube defects (ONTDs), anencephaly, and trisomies 21 and 18.

NEURAL TUBE DEFECTS

In the 1980s maternal serum screening programs became available to identify pregnancies at risk for ONTDs and anencephaly; 75-90% of ONTDs and >95% of anencephalics can be detected by elevated maternal serum alphafetoprotein (MSAFP) levels. MSAFP screening may also detect fetal abdominal wall defects.

The optimal time for NTD screening is 16-18 weeks’ gestation. Testing can be done between 15 and 20 weeks.  Ultrasound dating of the pregnancy reduces the false positive rate and increases the detection rate of ONTDs. The test is most accurate if the laboratory is also informed of maternal weight, race (Caucasian or Black/African American), presence of insulin dependent diabetes, number of fetuses, and family history of ONTD. Genetic counseling and additional testing such as targeted ultrasound examination and amniocentesis are recommended for pregnancies with an elevated MSAFP test result.

TRISOMIES 21 AND 18

Initially, MSAFP levels were used to modify a woman’s risk for trisomy 21 based on age alone, and screening was offered to women less than 35 years of age. Human chorionic gonadotrophin (hCG) and unconjugated estriol (uE3) were added to the screening protocol to increase the detection rate of Down syndrome in pregnancy. Multiple marker screening (MMS) using three analytes (MSAFP, hCG, uE3) to adjust a woman’s age-risk for Down syndrome raises the sensitivity to approximately 65%. This is often referred to as the “triple test.” Most recently, a fourth marker was added, dimeric inhibin-A (INH-A). This is referred to as the “quad or quadruple screen.” Using a second trimester Down syndrome cut-off risk of 1 in 270, the combination of maternal age, MSAFP, hCG, uE3, and INH-A detects approximately 75% of Down syndrome cases in women who are younger than 35 years with a positive screening rate of 5%, and over 80% of the Down syndrome fetuses in women 35 and older. In most cases of Down syndrome, the AFP and uE3 levels are lower, and hCG and INH-A levels are higher.

Amniocentesis

Amniocentesis is a diagnostic test that may be recommended by your health care provider following an abnormal first trimester screen or triple test result. Inherited or genetic concerns lead some parents to choose amniocentesis to determine if specific genetic disorders may be present in their baby.

How is amniocentesis performed?

An ultrasound is used as a guide to determine a safe location for the needle to enter the amniotic sac so the fluid may be safely removed. A sample of amniotic fluid is collected through the needle. The procedure takes about 45 minutes, although the collection of fluid takes less than five minutes. The amniotic fluid, which contains cells shed by the fetus, is sent to the laboratory for analysis. Results can take anywhere from a few days to a couple weeks to be returned.

When is amniocentesis performed? Amniocentesis is usually performed between 14 and 20 weeks. What does the amniocentesis test look for? Amniocentesis detects chromosome abnormalities, neural tube defects and genetic disorders. Down syndrome or Trisomy 21 is the most common chromosome abnormality. Genetic disorders include disorders like cystic fibrosis. The most common neural tube defect is spina bifida. Amniocentesis is occasionally used late in pregnancy to assess whether the baby's lungs are mature enough for the baby to breathe on his own. Amniocentesis also provides access to DNA for paternity testing prior to delivery. DNA is collected from the potential father and is compared to DNA obtained from the baby during amniocentesis. The results are accurate (99%) for determining paternity.

What do amniocentesis results mean?

Amniocentesis is a diagnostic test that detects chromosome abnormalities, neural tube defects and genetic disorders with high levels of accuracy (98-99%).

What are the risks and side effects to the mother or baby?

Although amniocentesis is considered to be a safe procedure, it is recognized as an invasive diagnostic test that does pose potential risks.

Miscarriage is the primary risk related to amniocentesis. The risk of miscarriage ranges from 1 in 200-300.

Although extremely rare, it is possible for the needle to come in contact with the baby. Great precautions are taken by using a sonogram to guide the needle away from the baby.

The mother may experience a sharp pain when the needle enters the skin and again when it enters the uterus. Following completion of the procedure, the mother may experience other side effects that include:

• Cramping
• Leakage of fluid
• Minor irritation around the puncture site

Contact your healthcare provider if these complications occur.

Chorionic villus sampling Chorionic villus sampling (CVS) is a form of prenatal diagnosis to determine genetic abnormalities in the fetus. It entails getting a sample of the chorionic villus (placental tissue) and testing it. It is generally carried out on pregnant women over the age of 35 and those abnormal first trimester screening or those whose offspring have a higher risk of congenital diseases. The advantage of CVS is that it can be carried out 10-12 weeks after the last period, earlier than amniocentesis (which is carried out at 15-18 weeks). Risks CVS is similar to amniocentesis in terms of miscarriage risk (0.2 to 0.3%). Apart from a risk of miscarriage, there is a risk of infection and amniotic fluid leakage. It is extremely important after having a CVS that the OB/GYN follow the patient closely.