Type of test Blood or urine
Normal findings
Low probability of fetal defects
Test explanation and related physiology
These tests are provided to pregnant women early in pregnancy to identify potential birth defects or serious chromosomal or genetic abnormalities. These screening tests may indicate the potential for the presence of fetal defects (particularly trisomy 21 [Down syndrome] or trisomy 18). They may also indicate increased risk for neural tube defects (e.g., myelomeningocele, spina bifida) or abdominal wall defects (omphalocele or gastroschisis).
Maternal screening is routinely offered to all pregnant women, usually in their second trimester of pregnancy. Patients must understand that this is a screening test, not a diagnostic test. If the screening test results are positive, more accurate definitive testing such as blood tests, chorionic villus sampling , or amniocentesis is recommended.
There are several variations of this test available:
• The double test measures two markers: human chorionic gonadotropin (hCG) and alpha-fetoprotein (AFP).
• The triple test (maternal triple screen test) measures three markers: hCG, AFP, and estriol . AFP is produced in the yolk sac and fetal liver. Unconjugated estriol and hCG are produced by the placenta.
• The quadruple test measures four markers: hCG, AFP, estriol, and inhibin A.
• The fully integrated screen test measures AFP, estriol, fetal nuchal translucency , beta and total hCG, and pregnancy-associated plasma protein-A.
The maternal triple screen test offers a 50% to 80% chance of detecting pregnancies with trisomy 21 compared with AFP alone, which has only a 30% chance of detection. The quadruple screen is now routinely recommended and is combined with fetal nuchal translucency (FTN). These tests are most accurately performed during the second trimester of pregnancy, more specifically between the 14th and 24th weeks (ideal 16th and 18th weeks). The use of ultrasound to accurately indicate gestational age improves the sensitivity and specificity of maternal serum screening.
First trimester screening for genetic defects is an option for pregnant women. This testing would include fetal nuchal translucency combined with the beta subunit of hCG, and PAPP-A. A low level of PAPP-A may indicate an increased risk for having a stillborn baby. These tests have detection rates comparable to standard second-trimester triple screening.
With trisomy 21, second trimester absolute maternal serum levels of AFP and unconjugated estriol are about 25% lower than normal levels, and maternal serum hCG is approximately two times higher than the normal hCG level. The results of the screening are expressed in multiples of median (MoM). AFP and urinary estriol (E3 ) values during pregnancies with trisomy 21 are lower than those associated with normal pregnancies, which means that values lower than the mean are lower than 1 MoM. The hCG value for trisomy 21 is higher than 1 MoM. The MoM, fetal age, and maternal weight are used to calculate the possible risk for chromosomal abnormalities. All of the previously named maternal screening tests are discussed elsewhere in this book. For the sake of thoroughness, inhibin A is discussed here.
Inhibin A levels in maternal serum remain relatively constant through the 15th to 18th week of pregnancy. Inhibin A is important in the control of fetal development. Maternal serum levels of inhibin A are twice as high in pregnancies affected by trisomy 21 as in unaffected pregnancies. The discovery of this fact led to the inclusion of inhibin A in the serum screening tests for trisomy 21. Inhibin A concentrations are significantly lower in women with normal pregnancies than in women with pregnancies that result in spontaneous abortions. Furthermore, circulating concentrations of inhibin A appear to reflect tumor mass for certain forms of ovarian cancer .
Procedure and patient care
Before
* Explain the procedure to the patient.
• Allow the patient to express her concerns and fears regarding the potential for birth defects.
During
• Most of these tests can be done with a venous blood sample in a red-top tube. The hCG and estriol can also be tested by collecting a urine sample.
After
* Provide the results to the patient during a personal consultation.
• Allow the patient to express her concerns if the results are positive.
* Assist the patient in scheduling and obtaining more accurate diagnostic testing if the results are positive.
Abnormal findings
Positive screening tests (trisomy 21, trisomy 18, neural tube defects, abdominal wall defects)
