## **Core Concept**
Prenatal diagnosis of haemophilia involves determining whether a fetus has inherited the genetic mutation responsible for the condition. Haemophilia A and B are caused by mutations in the genes that encode for clotting factors VIII and IX, respectively. Prenatal testing can help identify affected fetuses, allowing for informed decision-making.

## **Why the Correct Answer is Right**
The correct answer, **chorionic villus sampling (CVS)**, is a prenatal test that involves removing a small sample of cells from the placenta. These cells can be used for genetic testing, including analysis for haemophilia mutations. CVS can be performed early in pregnancy (between 10-12 weeks) and provides a definitive diagnosis. This allows for early decision-making and management planning.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Ultrasound is a valuable tool for monitoring fetal development but does not provide a genetic diagnosis of haemophilia. While it can help identify some fetal abnormalities, it is not specific for genetic conditions like haemophilia.
* **Option B:** Amniocentesis involves withdrawing a sample of amniotic fluid, which contains fetal cells that can be genetically tested. Although it can diagnose haemophilia, it is typically performed later in pregnancy (around 15-20 weeks) compared to CVS.
* **Option D:** Maternal serum screening is a blood test that screens for certain markers associated with an increased risk of genetic conditions. However, it does not provide a definitive diagnosis of haemophilia.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that CVS and amniocentesis carry a small risk of miscarriage. Therefore, the decision to undergo prenatal testing should be made after careful consideration and counselling. For families with a history of haemophilia, genetic counselling is crucial to discuss the risks and options.

## **Correct Answer:** . **Chorionic villus sampling**