**Core Concept**
Ferning of cervical mucus is a phenomenon where the cervical mucus forms a fern-like pattern when dried on a slide, indicating the presence of estrogen. This is a crucial aspect of the basal body temperature (BBT) and ferning method for predicting ovulation in fertility assessments.

**Why the Correct Answer is Right**
Ferning occurs due to the influence of estrogen on cervical mucus, causing it to become more alkaline and form a crystalline structure. Estrogen levels peak around ovulation, resulting in the characteristic ferning pattern. However, the ferning disappears after ovulation as progesterone levels rise, causing the cervical mucus to become more viscous and less conducive to ferning.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because ferning typically begins to disappear after ovulation, not before.
* **Option B:** This option is incorrect because ferning is not directly related to the timing of the menstrual cycle's first day, but rather to the hormonal changes throughout the cycle.
* **Option C:** This option is incorrect because ferning disappears after ovulation, not after the luteal phase begins.

**Clinical Pearl / High-Yield Fact**
Ovulation is typically predicted by the disappearance of ferning and a rise in basal body temperature, which can be used to estimate the timing of ovulation for fertility assessments.

**Correct Answer: D.**