## **Core Concept**
The first trimester of pregnancy is a critical period of fetal development and is defined as the first 13 weeks of gestation. It is a time of major organogenesis and development of the embryo.

## **Why the Correct Answer is Right**
The first trimester completes by the end of week 13, which corresponds to approximately 3 months of gestation. This period is crucial for the formation of major organs and body systems. Clinically, the first trimester is considered to end just before the 14th week of gestation.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option suggests completion by the end of week 10, which is too early for the completion of the first trimester.
* **Option B:** This option suggests completion by the end of week 20, which actually marks the end of the second trimester.
* **Option D:** This option suggests completion by the end of week 26, which is well into the third trimester.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the first trimester is a period of high risk for certain congenital anomalies due to organogenesis. Understanding the timeline of fetal development is crucial for providing appropriate prenatal care.

## **Correct Answer:** . 13 weeks