## **Core Concept**
The antiprogesterone compound RU-486, also known as mifepristone, works by blocking the action of **progesterone**, a hormone essential for maintaining pregnancy. It is used for medical termination of pregnancy by inducing uterine contractions and causing the embryo to be expelled. The effectiveness of RU-486 is highly dependent on the **gestational age**.

## **Why the Correct Answer is Right**
RU-486 is most effective for inducing abortion when the duration of pregnancy is **less than 7 weeks** or **up to 49 days** from the first day of the last menstrual period (LMP). This is because the drug's efficacy decreases as pregnancy advances, particularly after the embryo has implanted and the pregnancy is more established. At this early stage, the pregnancy is more dependent on progesterone for its maintenance, making it more susceptible to the effects of a progesterone receptor antagonist like mifepristone.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests that RU-486 is effective up to 28 days, which, although within the effective range, does not represent the maximum duration for which it is effective.
- **Option B:** This option suggests a broader window (up to 63 days), which exceeds the generally recommended duration for the use of mifepristone for medical abortion.
- **Option D:** This option suggests that RU-486 is effective up to 3 months, which is too late in pregnancy for mifepristone to be used as a single agent for abortion.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the effectiveness of mifepristone (RU-486) for inducing abortion decreases with advancing gestational age. It is **most effective when used within the first 7 weeks (up to 49 days) of pregnancy**. Additionally, its use is often combined with a prostaglandin analog (like misoprostol) to enhance efficacy.

## **Correct Answer:** . Up to 49 days.