**Core Concept**
The notching of the uterine artery waveform during early diastole is a sign of increased resistance to blood flow in the uteroplacental circulation. This phenomenon is associated with abnormal placentation and is a predictor of pregnancy complications such as preeclampsia and fetal growth restriction.

**Why the Correct Answer is Right**
The disappearance of early diastolic notching of the uterine artery waveform is indicative of a decrease in uteroplacental vascular resistance, which typically occurs by the 28th week of gestation. This decrease in resistance is thought to be due to the remodeling of the uteroplacental vasculature, which allows for increased blood flow to the placenta. The reduction in vascular resistance is mediated by the release of vasodilatory factors such as nitric oxide and prostacyclin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the notching of the uterine artery waveform typically persists beyond the 28th week of gestation in pregnancies complicated by preeclampsia or fetal growth restriction.
**Option B:** This option is incorrect because the disappearance of early diastolic notching is not a characteristic feature of normal pregnancy, but rather a sign of decreased uteroplacental vascular resistance.
**Option C:** This option is incorrect because the notching of the uterine artery waveform is not a feature of normal pregnancy, but rather a sign of abnormal placentation.

**Clinical Pearl / High-Yield Fact**
The disappearance of early diastolic notching of the uterine artery waveform by the 28th week of gestation is a critical milestone in pregnancy, as it indicates a decrease in uteroplacental vascular resistance and a reduction in the risk of pregnancy complications.

**Correct Answer:** C. 28th week.