**Core Concept**
Lactational amenorrhoea is a temporary cessation of menstruation in women who are breastfeeding. It is a natural form of contraception that relies on the hormonal changes that occur during lactation. The primary hormone responsible for this phenomenon is prolactin, which is secreted in response to suckling.

**Why the Correct Answer is Right**
During lactation, prolactin levels surge, causing a negative feedback inhibition on the hypothalamic-pituitary-gonadal axis. This leads to a decrease in the secretion of gonadotropin-releasing hormone (GnRH) and subsequently, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland. As a result, there is a decrease in estrogen production by the ovaries, which in turn inhibits the growth of the endometrium and prevents ovulation. This combination of factors results in amenorrhea, or the absence of menstruation.

**Why Each Wrong Option is Incorrect**
* **Option A:** While prolactin does play a role in lactational amenorrhea, it is not the sole cause. Other hormonal and physiological changes contribute to this phenomenon.
* **Option B:** Estrogen is actually decreased during lactational amenorrhea, not increased.
* **Option C:** Progesterone levels may be decreased or variable during lactational amenorrhea, but it is not the primary cause of this condition.

**Clinical Pearl / High-Yield Fact**
Lactational amenorrhea is a highly effective form of contraception when used correctly, with a failure rate of less than 2% in the first six months of exclusive breastfeeding.

**Correct Answer: C. Prolactin**