**Question:** The best time to do chorionic villous sampling is

A. First trimester (11-14 weeks)
B. Second trimester (16-20 weeks)
C. Third trimester (30-34 weeks)
D. None of the above

**Correct Answer:** B. Second trimester (16-20 weeks)

**Core Concept:** Chorionic villous sampling is a procedure used to obtain fetal tissue for genetic testing, typically to diagnose genetic disorders in the fetus. It is performed to assess the chromosomes and genetic material of the fetus to identify any abnormalities or genetic syndromes.

**Why the Correct Answer is Right:** Chorionic villous sampling is most appropriate during the second trimester (16-20 weeks) because the placenta is well-developed and the risk of miscarriage is minimal. At this stage, the placenta is mature enough to provide representative samples of the fetus.

**Why Each Wrong Option is Incorrect:**

A. First trimester (11-14 weeks): The placenta is not fully developed, making it difficult to obtain representative samples of the fetus. Additionally, the risk of miscarriage is higher in the first trimester due to the fragile nature of the placenta.

C. Third trimester (30-34 weeks): At this stage, the placenta is less suitable for obtaining accurate genetic material due to the rapid fetal growth and maturation. Moreover, the procedure carries a higher risk of prematurity and complications for both the mother and fetus.

D. None of the above: While it is true that chorionic villous sampling can be performed at different gestational stages, the most appropriate time remains the second trimester (16-20 weeks) due to the well-developed placenta and lower risk of miscarriage.

**Clinical Pearl:** Chorionic villous sampling is typically performed under ultrasound guidance to ensure the correct placental location and minimize complications. It is typically done during routine prenatal visits and is usually combined with other diagnostic tests, such as amniocentesis and noninvasive prenatal testing (NIPT). The sample obtained can be used for various genetic tests, including karyotyping and microarray analysis. Knowing the correct gestational age is crucial for a successful and safe procedure.