A decrease in alpha-fetoprotein in amniotic fluid or maternal serum is suggestive of which condition?

A decrease in alpha-fetoprotein (AFP) in amniotic fluid or maternal serum is primarily associated with Down syndrome (trisomy 21). AFP is a protein produced by the fetal liver, and its levels can indicate certain fetal conditions during pregnancy.

In the context of Down syndrome, decreased levels of AFP are part of a broader pattern of markers used in first-trimester screening tests. These tests typically assess multiple biometrics, including nuchal translucency and levels of other markers, which together help estimate the risk of chromosomal abnormalities. Lower AFP levels are indicative of the likelihood of conditions like Down syndrome because they diverge from the expected patterns observed in healthy pregnancies.

Other conditions such as neural tube defects generally present with elevated levels of AFP, as the protein leaks into the amniotic fluid from the defect. In multiple gestations, AFP levels tend to be higher due to the presence of more than one fetus. Congenital adrenal hyperplasia may also affect other hormonal levels but is not specifically characterized by decreased AFP.

Therefore, a decrease in alpha-fetoprotein serves as an important indicator in prenatal screening for Down syndrome.