The triple screen is a blood screening test that is done in the second trimester of pregnancy to provide a risk estimate for the occurrence of Down syndrome in the fetus. It consists of three tests, the results of which are combined with the maternal age to derive the risk:
- MS-AFP (maternal serum alpha-fetoprotein)
- Human chorionic gonadotropin (hCG)
- Unconjugated estriol (uE3)
Value of Triple Screen
Several studies have pointed out the lack of major differences in the occurrence of Down syndrome in pregnancies which are screen-positive compared to screen-negative groups. This underlines the essential ‘screening’ nature of the test, in that it doesn’t necessarily mean the baby will be affected. In fact, the most common cause for a positive triple screen is wrong pregnancy dates, which means the baby is older than expected by the dates given by the mother. Again, a negative screen does not always mean the baby will be completely normal, because it doesn’t detect all types of congenital defects.
The triple test may be offered to high-risk women only, such as:
- Older mothers in whom the risk of birth defects, specifically Down syndrome, is higher
- Women with a prior personal or family history of babies with birth defects
Timing of the Triple Screen
The triple screen is most valuable when performed between 16-18 weeks of pregnancy, but may be done anytime between 15 and 22 gestational weeks.
Abnormal Test Values
If the triple screen results are positive, several options are possible. In some cases, an immediate ultrasound scan may be ordered to confirm the age of the fetus as well as to rule out or detect neural tube defects (NTD), renal anomalies, cardiac defects, or gastrointestinal anomalies.
Another test that may be recommended is amniocentesis, in which 15-20 mL of amniotic fluid is gently withdrawn from the fluid in the sac surrounding the fetus in the womb.
AFP is a fetal protein which makes up the bulk of fetal serum proteins. It is produced mainly by the yolk sac and liver in the fetus. It is initially low but rises by about the 14th week. At this point fetal AFP crosses the placenta to enter the maternal blood stream.
The level of AFP rises from about 0.2 ng/mL before pregnancy to 250 ng/mL at 32 weeks. Increased leakage of AFP occurs in neural tube defects, gastrointestinal tract anomalies, and other fetal conditions, as well as in multiple pregnancies and wrong dates (when the fetus is older than expected by the given pregnancy dates). Low levels of AFP are seen in Down syndrome and other trisomies.
Unconjugated Estriol (uE3)
Estriol is the chief estrogen circulating during pregnancy, and is produced by the fetus and placenta. It is partly conjugated to increase its excretion via urine by making it water-soluble. Its levels rise with gestational term. Low levels are found in Down syndrome and trisomy 18, as well as with wrong dates (when the fetus is younger than the pregnancy dates suggest), missed abortion, and several other congenital syndromes.
Human Chorionic Gonadotropin (hCG)
hCG is a glycoprotein composed of two subunits, one alpha and one beta. The alpha subunit is identical to those found in other hormones such as luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The beta subunit resembles that of LH and, to a lesser extent, those of FSH and TSH. It is responsible for the unique functional properties of hCG. Its synthesis begins early in pregnancy and maintains the function of the corpus luteum, viz., progesterone production and pregnancy maintenance, during the first trimester, until the placenta matures sufficiently to begin taking over this function after 13 weeks. High beta-hCG levels occur in Down syndrome, younger-than-expected fetuses (because of wrong dates), multiple gestations and triploid pregnancies, as well as in missed abortions and hydrops fetalis. Low levels are seen in trisomy 18.
Like many other screening tests, the triple screen is based upon the Bayes theorem, and helps derive a risk estimate by taking the expected maternal age at delivery in combination with a likelihood ratio. When the risk is higher than a predetermined level, a diagnostic test such as amniocentesis or chorionic villus biopsy could be offered to make the true diagnosis. The cells obtained by these latter techniques may be subjected to cell culture for karyotyping, or fluorescent in situ hybridization (FISH).
Current Relevance of the Triple Screen
The triple test was first used in 1988, and many other tests have now entered the scene. It is now clear that the serum levels of these analytes are different in singleton and multiple gestations, which could lead to a higher number of unnecessary amniocentesis procedures in women with normal twin pregnancies. Thus, the levels of suspicion for each of the test constituents needs to be adjusted depending on the type of pregnancy.
Again, maternal weight and age influence the risk estimate, independent of the actual serum levels of these chemicals. Maternal weight changes are related to many factors including multiple pregnancy. Thus, a separate model may be needed to make the test more accurate in the context of higher-order gestations. More efficient and non-invasive tools have been evolved over the past couple of decades, which may be more widely adopted in future.