**Question:** All except one is true regarding progestin only contraceptives

**Core Concept:** Progestin-only contraceptives are hormonal methods that primarily suppress ovulation by inhibiting the luteinizing hormone (LH) surge and thickening the cervical mucus, preventing sperm from reaching the uterus.

**Why the Correct Answer is Right:**

1. **A. Progestin-only contraceptives do not affect the endometrium:** This statement is correct, as they primarily act on the hypothalamic-pituitary-ovarian axis, preventing the LH surge and subsequent ovulation.

2. **B. Progestin-only contraceptives do not contain estrogen:** This is true, as they are made up of synthetic progestogens only, unlike combined hormonal contraceptives which contain both estrogen and progestin.

3. **C. Progestin-only contraceptives do not prevent implantation:** They do not interfere with the implantation process, but they do prevent fertilization and ovulation, thus reducing the chances of pregnancy.

4. **D. Progestin-only contraceptives increase uterine bleeding:** While this may be observed in some patients, the primary function of progestin-only contraceptives is to prevent ovulation and fertilization, not to induce menstruation.

**Why Each Wrong Option is Incorrect:**

1. **A. Progestin-only contraceptives affect the endometrium:** This statement is incorrect because progestin-only contraceptives primarily focus on preventing ovulation and do not significantly affect the endometrial lining.

2. **B. Progestin-only contraceptives contain estrogen:** This is incorrect as these contraceptives consist solely of synthetic progestogens, unlike combined hormonal contraceptives with estrogen and progestin components.

3. **C. Progestin-only contraceptives prevent implantation:** This statement is false because progestin-only contraceptives prevent fertilization and ovulation, not implantation. They do not interfere with the implantation process.

4. **D. Progestin-only contraceptives do not increase uterine bleeding:** This statement is incorrect as some users may experience altered menstrual patterns or heavier menstrual bleeding due to changes in endometrial and uterine blood vessel constriction. However, the primary function of progestin-only contraceptives is to prevent ovulation and fertilization, not to induce menstruation.

**Clinical Pearl:** In summary, progestin-only contraceptives primarily focus on preventing ovulation and fertilization, rather than affecting the endometrium or inducing menstruation. Some users may experience altered bleeding patterns due to hormonal changes, but this is not their primary mechanism of action.