Which of the following is the investigation of choice in a pregnant lady at 18 weeks of pregnancy with pasthistory of delivering a baby with Down’s syndrome:
**Question:** Which of the following is the investigation of choice in a pregnant lady at 18 weeks of pregnancy with a past history of delivering a baby with Down's syndrome:
A. Chorionic Villus Sampling (CVS)
B. Amniocentesis
C. Serum screening
D. Ultrasound
**Correct Answer:** B. Amniocentesis
**Core Concept:**
In pregnant women, the diagnosis of Down syndrome (trisomy 21) is crucial to ensure appropriate prenatal and postnatal care. Prenatal tests are further classified into invasive and non-invasive tests. Invasive tests involve sampling the amniotic fluid or the placenta, while non-invasive tests rely on blood tests or ultrasound.
**Why the Correct Answer is Right:**
Amniocentesis is an invasive prenatal test that involves the collection of amniotic fluid cells for analysis. In this case, amniocentesis is preferred as it provides a more accurate diagnosis of Down syndrome compared to non-invasive tests like serum screening or ultrasound. Amniocentesis can detect trisomy 21 with a sensitivity of around 99% and a specificity of 99%, making it the gold standard for diagnosing Down syndrome in high-risk pregnancies.
**Why Each Wrong Option is Incorrect:**
A. Chorionic Villus Sampling (CVS) is another invasive test but is performed earlier in pregnancy (10-14 weeks) than amniocentesis (15-20 weeks). Although CVS has a higher diagnostic yield for Down syndrome, it is less specific than amniocentesis due to the higher risk of miscarriage (1-2%) associated with CVS.
C. Serum screening involves measuring specific proteins and hormones in the mother's blood to assess the risk of Down syndrome. While serum screening can be useful in the initial assessment of a high-risk pregnancy, it is less specific than invasive tests like amniocentesis due to false-positive and false-negative results.
D. Ultrasound is a non-invasive test used to observe fetal anatomical features and growth, but it is not a diagnostic test for Down syndrome. Ultrasound can provide important information about the fetus and confirm the pregnancy status, but it does not diagnose Down syndrome itself.
**Clinical Pearl:**
In cases where a pregnant woman has a history of delivering a child with Down syndrome, amniocentesis remains the gold standard for the accurate diagnosis of Down syndrome at 15-20 weeks of pregnancy. Amniocentesis provides a more precise diagnostic result compared to serum screening, ultrasound, or CVS, reducing the risk of miscarriage with an appropriate technique and timing.