Amniocentesis is done at
**Core Concept**
Amniocentesis is a prenatal diagnostic test that involves the withdrawal of a small sample of amniotic fluid from the amniotic sac surrounding the fetus. This procedure is typically performed to diagnose chromosomal abnormalities, genetic disorders, or infections.
**Why the Correct Answer is Right**
Amniocentesis is usually performed between the 15th and 20th week of gestation, specifically at 16-18 weeks. This timing allows for the detection of alpha-fetoprotein (AFP) levels, which can indicate potential neural tube defects or other fetal anomalies. The test also involves the analysis of fetal cells in the amniotic fluid, which can provide valuable information about the fetus's genetic makeup.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because amniocentesis is typically not performed before the 15th week of gestation, as the risk of complications such as preterm labor or fetal membrane rupture is higher at this stage.
**Option B:** This option is incorrect because amniocentesis is not usually performed after the 20th week of gestation, as the risk of complications and the accuracy of the test decrease with advancing gestational age.
**Option C:** This option is incorrect because amniocentesis is not typically performed before the 16th week of gestation, as the fetal cells in the amniotic fluid are not mature enough for analysis at this stage.
**Clinical Pearl / High-Yield Fact**
It is essential to note that amniocentesis carries a small risk of complications, including preterm labor, fetal membrane rupture, and infection. Therefore, it is typically reserved for women at high risk of chromosomal abnormalities or genetic disorders.
**Correct Answer: C**. 16-18 weeks of gestation.