## Core Concept
Superfoetation refers to the formation of a fetus while another fetus is already present in the uterus, typically resulting from a second ovum being fertilized by sperm after another embryo has already implanted. This concept is crucial in obstetrics and gynecology.

## Why the Correct Answer is Right
The correct answer, , accurately describes superfoetation as the development of a second fetus, fertilized by a separate act of sexual intercourse, while another fetus is already present in the uterus. This phenomenon usually occurs when a woman releases multiple ova at different times, and each ovum is fertilized and implants at a different time. This can happen in women who have irregular menstrual cycles or who conceive while already pregnant.

## Why Each Wrong Option is Incorrect
* **Option A:** - This option does not accurately describe superfoetation.
* **Option B:** - This option does not accurately describe superfoetation.
* **Option C:** - This option does not accurately describe superfoetation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that superfoetation is extremely rare in humans. For superfoetation to occur, a woman must ovulate again after becoming pregnant, which is not common. This phenomenon highlights the complexities of human reproduction and the potential for multiple gestations.

## Correct Answer Line
**Correct Answer: D. Fertilization of second ovum after first has implanted**