PRENATEL & MATERNAL SCREENING
This page describes the prenatal testing a pregnant mother might undergo, including routine testing, screening tests, diagnostic tests, and tests in late pregnancy.
- A number of tests are done routinely during pregnancy, including blood and urine tests, a Pap smear and ultrasounds.
- Screening tests are offered to pregnant women when there is a risk or suspicion that the baby may not be developing properly.
- Diagnostic tests are used to confirm the presence of a particular abnormality in the unborn baby.
There are many tests that can be done throughout pregnancy to make sure that you and your baby are progressing well. Some of these tests are done routinely and others are done if there is concern about the well-being of the baby.
A number of tests are done routinely during pregnancy. These include urine tests, blood tests, a Pap smear, an ultrasound, and tests for gestational diabetes and Streptococcus B.
During the first trimester, your doctor can order two types of blood tests called a sequential integrated screening test and a serum integrated screening.
They’re used to measure the levels of certain substances in your blood, namely, pregnancy-associated plasma protein-A and a hormone called human chorionic gonadotropin. Abnormal levels of either mean there’s a higher risk of a chromosome abnormality.
At your first prenatal visit, your blood may also be tested to see if you’re immunized against rubella and to screen for syphilis, hepatitis B, and HIV. Your blood will also likely be checked for anemia.
A blood test will also be used to check your blood type and Rh factor, which determines your Rh compatibility with your growing baby. You can be either Rh-positive or Rh-negative.
Most people are Rh-positive, but if you’re Rh-negative, your body will produce antibodies that will affect any subsequent pregnancies.
When there’s an Rh incompatibility, most women will be given a shot of Rh-immune globulin at 28 weeks and again a few days after delivery.
Incompatibility occurs if a woman is Rh-negative and her baby is Rh-positive. If the woman is Rh-negative and her baby’s positive, she’ll need the shot. If her baby is Rh-negative, she won’t.
Note: There’s not a noninvasive way to determine your baby’s blood type until after they’re born.
First Trimester Screening – Double Marker Test (10 to 13+6 weeks)
Early detection and higher sensitivity.
The parents and the doctors indeed want to know as early as possible about the possibility of genetic anomalies. The earliest window available for getting such information is between 10 to 14 weeks (or 13 weeks + 6 days to be more precise) through the double market test and NT scan.
Double Marker – Two Biochemical Parameters
It has been found that there is a strong correlation between certain biochemical parameters and the fetal abnormalities. Two such biochemical parameters which are relevant in the first trimester are Free Beta-human chorionic gonadotropin (Free B-HCG) and ‘Pregnancy-Associated Plasma Protein (PAAP-A). By combining the result of the NT Scan and the two biochemical parameters (Dual Maker or Double Marker) into a sophisticated risk calculation algorithm or software, we can provide a reliable indication for the risks of Down Syndrome (Trisomy 21), Edwards Syndrome (Trisomy 18) and Patau Syndromes (Trisomy 13). The First Trimester Screening presents significant advantages over other tests because of higher detection rates (90-95% with 5% false positive rates), and the availability of information at an early stage of pregnancy. ~ (Reference: Fetal Medicine Foundation).
Second Trimester Screening – Triple/Quadruple Test (16 to 21 weeks)
The Second Trimester Screening is done between 16th to 20th weeks of the pregnancy. The Triple and Quadruple tests offered as part of the second-trimester screening provide an option to the couple who has missed the first-trimester screening. The first-trimester screening is preferable to second-trimester screening due to its high sensitivity.
Triple Marker Test
The Triple Marker Test measures the levels of three important substances in the placenta: Alpha-fetoprotein (AFP), Human chorionic gonadotropin (HCG) and Unconjugated Estriol (uE3). The abnormal levels of these markers can detect the possibility of fetus having: Trisomy 13,18 or 21 and Open Neural Tube Defects (ONTD) such as Spinabifda, anencephaly etc.
Quadruple Marker Test
The Quadruple marker test combines the three markers of the triple marker test (AFP, hCG, uE3), along with an additional marker Inhibin-A. This test is useful in identifying the Trisomy 13, Trisomy 18, Trisomy 21 and Open Neural Tube Defects (ONTD).
Interpreting the Prenatal Screening Tests
The prenatal screening tests are meant to provide an early indication about the possibility of common genetic anomalies such as Down Syndrome or Trisomy 13, 18 etc. These tests typically compare the probability of a genetic defect vis-a-vis the age related risk. Maternal age is known to have a very well established correlation with several genetic disorders. However, advanced maternal age (mothers > 35 years) is no longer considered a definitive cut-off since many genetic abnormalities have been observed in mothers of much younger age.
The information available through screening tests helps classify the women into High Risk and Low Risk categories. These results need to be confirmed using confirmatory tests such as Karyotyping or Microarray.
The High Risk pregnant women are advised to undergo invasive diagnostic procedures like Amniocentesis or Chorionic Villus Sampling (CVS). A low risk in the screening tests does not rule out the genetic abnormalities, and the gynecologist or fetal medicine specialist may still advice an invasive diagnostic procedure depending upon the conditions of each specific case. In certain countries (e.g. USA), all women are offered the option of the invasive procedures.