Which of the following is the investigation of choice in a pregnant lady at 18 weeks of pregnancy with past history of delivering a baby with Down’s syndrome.
**Core Concept**
Non-invasive prenatal testing (NIPT) is a screening test that analyzes cell-free DNA in the maternal blood to detect chromosomal abnormalities, such as Down syndrome, Trisomy 13, and Trisomy 18. This test is particularly useful in the second trimester of pregnancy, around 18 weeks.
**Why the Correct Answer is Right**
The correct answer is **Non-invasive prenatal testing (NIPT)** because it is a safe and accurate method to screen for chromosomal abnormalities in the fetus. At 18 weeks of gestation, NIPT can analyze the cell-free DNA in the maternal blood to detect an increased risk of Down syndrome. This test is particularly useful in women with a history of delivering a baby with Down syndrome, as it can help identify those at high risk and allow for further diagnostic testing, such as amniocentesis or chorionic villus sampling (CVS).
**Why Each Wrong Option is Incorrect**
**Option A:** **Ultrasound scan** is not the investigation of choice for detecting chromosomal abnormalities such as Down syndrome. While ultrasound can provide valuable information about fetal anatomy and growth, it is not a reliable method for detecting chromosomal abnormalities.
**Option B:** **Chorionic villus sampling (CVS)** is a diagnostic test that involves removing a sample of cells from the placenta to diagnose chromosomal abnormalities. However, CVS is typically performed earlier in pregnancy, around 10-12 weeks, and carries a slightly higher risk of complications compared to NIPT.
**Option C:** **Amniocentesis** is a diagnostic test that involves removing a sample of amniotic fluid to diagnose chromosomal abnormalities. However, amniocentesis is typically performed later in pregnancy, around 15-20 weeks, and carries a slightly higher risk of complications compared to NIPT.
**Clinical Pearl / High-Yield Fact**
NIPT is a safe and accurate screening test for chromosomal abnormalities, and it can be performed as early as 10 weeks of gestation. However, it is essential to note that NIPT is a screening test, not a diagnostic test, and a positive result should be confirmed with further diagnostic testing.
**Correct Answer:** A. Non-invasive prenatal testing (NIPT).