Down Syndrome Screening in Pregnancy
The 1980s were a time of research and development of "markers" in the maternal blood that would hopefully point to an increased risk of certain fetal defects. It was noted that mothers carrying a fetus with an open spinal defect (neural tube defect) have higher levels of a substance known as alpha-fetoprotein (AFP). Shortly thereafter, this same substance was noted to be decreased in mothers carrying a baby affected by Down syndrome. Soon, other markers were also found in pregnancies affected with fetal Down syndrome, including:
-- Lower levels of unconjugated estriol (uE3)
-- Increased human chorionic gonadotropin (hCG)
-- Increased levels of inhibin A (IH-A)
This research stimulated the development of the three-marker and now the four-marker screen offered in the early second trimester of pregnancy, typically between weeks 15 and 21 from the first day of the last period.
We all understand that it can be devastating to hear very close to the halfway point in pregnancy that you have a higher than average risk for genetic defects. Research is continuing today to find another marker that could be offered in the first trimester. To that end, the nuchal translucency ultrasound combined with the pregnancy-associated plasma protein A (PAPP-A) was developed and can be performed late in the first trimester, at about 11 to 13 weeks. It has been noted that babies with certain genetic abnormalities, specifically Down syndrome and trisomy 18, had abnormal accumulations of fluid at the back of the neck, and after many trials it was discovered that this area could be targeted by ultrasound and quantified, or measured. Thickness greater than three millimeters was associated with an affected fetus, but still the test lacked the required sensitivity — not all babies with such fluid had the condition.