**Question:** Best indicator of gestational age in first trimester is -

A. Crown-rump length (CRL)
B. Fetal heart rate
C. Fetal biparietal diameter (BPD)
D. Amniotic fluid index (AFI)

**Correct Answer:** A. Crown-rump length (CRL)

**Core Concept:**
The assessment of gestational age is essential in prenatal care as it helps in determining fetal growth, development, and overall health. In the first trimester (weeks 1-13), crown-rump length (CRL) is considered the best indicator of gestational age due to its accuracy and ease of measurement.

**Why the Correct Answer is Right:**
CRL is a reliable indicator of gestational age in the first trimester because it directly correlates with fetal crown-to-rump length, which is proportional to overall fetal size and growth. Measurement of CRL allows clinicians to estimate gestational age with a high degree of accuracy, which is crucial for early detection of potential complications and proper prenatal care planning.

**Why Each Wrong Option is Incorrect:**

A. Fetal heart rate (FHR) is not a reliable indicator of gestational age in the first trimester as it varies significantly among fetuses and does not reflect the overall fetal size.

B. Fetal biparietal diameter (BPD) is not the best indicator of gestational age in the first trimester. While BPD is an important measurement in later gestations, it is less accurate earlier on due to limited fetal size and structure development.

C. Amniotic fluid index (AFI) is not relevant in the first trimester, as AFI is determined by measuring the depth of amniotic fluid in four quadrants, which is not possible before fetal limbs and body parts are distinctly visible (usually after 28 weeks).

**Clinical Pearl:**

In practice, measuring CRL is crucial during the first trimester (weeks 1-13) to accurately estimate gestational age and provide appropriate prenatal care. Assessing fetal growth and development at this stage aids in early detection of potential complications, such as preterm labor or fetal growth restriction, allowing for timely interventions and improved perinatal outcomes.