Which one of the following congenital malformations of the fetus can be diagnosed in the first trimester by ultrasound?
**Core Concept**
The first trimester is a critical period for fetal development, and ultrasound examination can detect certain congenital malformations. The nuchal translucency (NT) scan, performed between 11 and 14 weeks of gestation, assesses the thickness of the fluid-filled space at the back of the fetus's neck. Abnormal NT thickness is associated with an increased risk of chromosomal abnormalities and congenital heart defects.
**Why the Correct Answer is Right**
A thickened NT is often an indicator of aneuploidy, particularly Down syndrome (trisomy 21). The mechanism behind this association is not fully understood, but it is believed to be related to the impaired development of the lymphatic system. The increased NT thickness may be due to the accumulation of fluid in the subcutaneous tissue, which can be seen on ultrasound. The NT measurement is an important marker for fetal aneuploidy, and a thickened NT is considered a soft marker for Down syndrome.
**Why Each Wrong Option is Incorrect**
**Option A:** Omphalocele is a congenital defect where the intestines or other organs protrude through the navel, but it is typically diagnosed later in pregnancy, usually after 20 weeks.
**Option B:** Gastroschisis is a congenital defect where the intestines protrude through a hole in the abdominal wall, but it is also typically diagnosed later in pregnancy, usually after 20 weeks.
**Option C:** Trisomy 18 (Edwards syndrome) is a chromosomal abnormality that can be associated with a shortened NT, but it is not typically diagnosed in the first trimester.
**Clinical Pearl / High-Yield Fact**
A thickened NT is not the only marker for Down syndrome, and a combination of ultrasound markers and serological tests should be used to increase the diagnostic accuracy.
**Correct Answer:** C. Trisomy 21 (Down syndrome).