Alpha-fetoprotein ( AFP ) is a protein made by fetal liver. There is a lot of AFP in the baby’s blood, and a small amount crosses the amniotic membrane and passes into the mother.
While AFP is high in mothers of babies with neural tube defects, it tends to be lower than normal when a woman is carrying a baby with Down’s syndrome or trisomy 18.
It should be noted that some AFP tests show high amounts of AFP for no known reason. Even if ultrasound and amniocentesis confirm a normal pregnancy, these babies have a higher than average risk of toxemia, preterm birth, decreased amniotic fluid, and being small for their gestational date. The high AFP should alert the practitioner to keep a closer watch on such pregnancies. Whether this reflects a defect in the placental-uterine connection is currently being researched.
Other Blood Screening Tests
There are three other blood tests that are commonly used to screen pregnancies that are at risk for Down’s syndrome and trisomy 18. HCG (human chorionic gonadatropin) is a hormone made by the placenta that helps the fetus grow. When a baby has Down’s syndrome, there is more HCG than usual; when trisomy 18 is present, there is less HCG than usual.
During pregnancy, the placenta makes a protein called free Beta HCG. This is the most specific biochemical marker for Down’s syndrome. Free Beta HCG is high when the baby has Down’s, and low when the baby has trisomy 18.
First trimester screening and diagnosis of chromosomal problems. In 1999 T. Hallaghan, J. Macri and myself put together our research data on 1 st trimester fetal diagnosis.
The prospect of maternal serum screening for fetal aneuploidy in the first trimester of pregnancy, with subsequent karyotyping when indicated, offers distinct advantages over current protocols and has inspired a lot of research lately. Under the current system, whereby screening is not undertaken until the second trimester, pregnant women have little time for decision-making regarding possible termination of an affected fetus. An earlier warning would offer women valuable time to make decisions, as well as an opportunity for earlier and safer termination of the pregnancy if desired.
When combined, free B-hCG and PAPP-A can detect 63% of Down syndrome cases in the first trimester, with a fixed false-positive rate of 5%.
Patients found to be at increased risk for carrying a child with Down syndrome can be offered fetal karyotyping by chorionic villi sampling. In the second trimester, AFP testing is still indicated to screen for neural tube defects, and multimarker Down syndrome testing can be performed as well. However, two important factors should be considered when weighting the merits of second-trimester rescreening in women who have undergone prior first-trimester testing: 90% of Down syndrome cases are detected with first-trimester screening (false-positive rate, 5%); and second-trimester screening will incur an additional 5% false-positive rate while detecting only about 60% of the remaining 10% of cases.
CONCLUSION
First-trimester Down syndrome screening using a combination of biochemical markers (maternal serum free B-hCG and PAPP-A) and a biophysical parameter (fetal nuchal translucency) can achieve 90% detection, with a false-positive rate 5%. This exceeds the detection rate offered by any second-trimester protocol, and offers a majority advantage of earlier diagnosis.