Karyotyping of the fetus can be done routinely through all of the following invasive method except:
The main methods I remember are amniocentesis and chorionic villus sampling (CVS). Amniocentesis involves taking amniotic fluid, usually around 15-20 weeks, and the cells from the fluid can be cultured for karyotype. CVS is done earlier, around 10-12 weeks, by taking a sample of the placental tissue. Both of these are standard and commonly used.
Then there's another method called percutaneous umbilical cord blood sampling (PUBS), also known as cordocentesis. This is less common because it's riskier and usually done later in pregnancy, around 18 weeks or more. It's used when other tests are inconclusive. So PUBS might not be considered a routine method compared to amniocentesis and CVS.
Another possible option could be fetal skin biopsy, but I don't recall that being a standard procedure. Maybe it's a distractor. The question asks which is not routinely used, so if the options include PUBS, that's the answer. Alternatively, maybe something like fetal blood sampling (PUBS) is considered not routine. Let me verify.
Yes, karyotyping can be done through amniocentesis, CVS, and PUBS. But PUBS is not as routine due to higher risks like fetal loss. So if the options include PUBS as an option, that's the correct answer. The question states "except," so the exception is the one that's not routinely used. The other methods (amniocentesis and CVS) are standard. So the answer should be PUBS.
**Core Concept**
Karyotyping of a fetus involves analyzing chromosomes to detect abnormalities. Invasive methods like amniocentesis, chorionic villus sampling (CVS), and percutaneous umbilical cord blood sampling (PUBS) are used, but their routine use depends on gestational age, risk profiles, and diagnostic accuracy.
**Why the Correct Answer is Right**
**Option D (Percutaneous umbilical cord blood sampling/PUBS)** is the correct answer because it is not routinely used for fetal karyotyping. PUBS involves sampling blood from the umbilical cord, typically after 18 weeks, and carries higher risks (e.g., fetal loss, preterm labor) compared to amniocentesis or CVS. It is reserved for cases where earlier tests are inconclusive or when urgent genetic results are needed.
**Why Each Wrong Option is Incorrect**
**Option A (Amniocentesis):** A routine procedure at 15–20 weeks, amniocentesis is standard for karyotyping due to its high diagnostic yield and lower risk profile.
**Option B (Chorionic villus sampling/CVS):** Performed at 10–12 weeks, CVS is a first-trimester routine test for chromosomal abnormalities.
**Option C (Fetal skin biopsy):** Though theoretically possible, it is not a standard or routine method for karyotyping due to technical challenges and risks.
**Clinical Pearl / High-Yield Fact