AFP is increased in maternal serum in all except:
**Core Concept**
Alpha-fetoprotein (AFP) is a protein produced by the yolk sac and fetal liver during pregnancy. Maternal serum AFP levels are used as a screening test for fetal abnormalities, particularly neural tube defects and Down syndrome.
**Why the Correct Answer is Right**
AFP is a major plasma protein in the fetus, and its levels increase during pregnancy. Elevated maternal serum AFP levels can indicate fetal anomalies such as neural tube defects, abdominal wall defects, and certain genetic conditions like Down syndrome. The mechanism behind this association is not fully understood, but it is believed to be related to the disruption of the normal placental-fetal barrier.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because AFP is indeed increased in maternal serum in cases of multiple gestations, such as twins or triplets. This is due to the increased production of AFP by multiple fetuses.
**Option B:** This option is incorrect because AFP is decreased in maternal serum in cases of Down syndrome. However, this option is not among the choices provided.
**Option C:** This option is incorrect because AFP is increased in maternal serum in cases of fetal abdominal wall defects, such as omphalocele or gastroschisis. The increased AFP levels are thought to reflect the disruption of the normal placental-fetal barrier.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that a normal maternal serum AFP level does not rule out fetal abnormalities, and further diagnostic testing is often necessary to confirm the presence or absence of a condition.
**Correct Answer: A. Multiple gestations, such as twins or triplets, typically have higher AFP levels due to the increased production of AFP by multiple fetuses.**