Ultrasound is done in 1st trimester for all except: March 2009
**Core Concept**
Ultrasound in the 1st trimester is crucial for dating the pregnancy, assessing fetal viability, and detecting potential congenital anomalies. It involves transabdominal and transvaginal approaches to visualize the embryo, yolk sac, and fetal pole.
**Why the Correct Answer is Right**
In the 1st trimester, ultrasound is primarily used to confirm pregnancy, estimate gestational age, and detect multiple pregnancies. It is also used to assess fetal viability by identifying fetal heart activity, the presence of a yolk sac, and the development of the embryo. Transvaginal ultrasound is more sensitive for detecting early pregnancy and is often used in conjunction with transabdominal ultrasound.
**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is commonly performed in the 1st trimester for all pregnant women, making it an essential tool for early pregnancy assessment.
**Option B:** Ultrasound in the 1st trimester is not typically used for fetal sex determination, which is usually done later in the pregnancy.
**Option C:** Ultrasound in the 1st trimester is not used for measuring fetal biometry, which is typically done in the 2nd and 3rd trimesters.
**Option D:** Ultrasound in the 1st trimester is not used for detecting fetal anomalies, which is typically done in the 2nd trimester with a detailed anomaly scan.
**Clinical Pearl / High-Yield Fact**
The "double decidual sign" on ultrasound, visible in the 1st trimester, is a reliable indicator of early pregnancy and can be used to confirm a viable intrauterine pregnancy.
**Correct Answer: B. Fetal sex determination.**