## **Core Concept**
The establishment of placental circulation is a critical step in early embryonic development, marking the transition from reliance on maternal uterine secretions for nutrition to a more direct exchange of gases, nutrients, and waste products. This process involves the implantation of the blastocyst and the development of the placenta.

## **Why the Correct Answer is Right**
Placental circulation is established around the 21st day after fertilization. This process begins with the implantation of the blastocyst into the uterine wall, which occurs about 6-7 days post-fertilization. Following implantation, the trophoblast differentiates into cytotrophoblast and syncytiotrophoblast. The syncytiotrophoblast invades the uterine lining, and by around 21 days post-fertilization, the primitive heart starts to pump blood through the chorionic villi, establishing a basic form of placental circulation. This allows for the exchange of nutrients and waste between the mother and the embryo.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify a timeframe that aligns with known developmental milestones related to placental circulation.
- **Option B:** This option suggests a timeframe that might be too early for the full establishment of functional placental circulation.
- **Option D:** This option suggests a timeframe that is later than necessary for the initial establishment of placental circulation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that by around the 21st day after fertilization, the embryonic heart starts to beat and pump blood, and the basic structure of the placenta and its circulation begins to function. This period is critical for the transition from a pre-embryonic to an embryonic stage of development.

## **Correct Answer:** C. 21 days.