**Core Concept**
The S/D ratio of the umbilical artery is a measure of fetal well-being, reflecting placental resistance and fetal blood flow. It is calculated by dividing the systolic velocity by the diastolic velocity in the umbilical artery Doppler waveform.

**Why the Correct Answer is Right**
As gestational age increases, the S/D ratio typically decreases due to decreased placental resistance and increased fetal blood flow. This decrease in S/D ratio is a normal adaptation to increasing gestational age, indicating a healthy fetus with optimal placental function. The decrease in S/D ratio is thought to be mediated by the increased production of vasodilatory substances such as prostaglandins and nitric oxide, which relax the smooth muscle cells in the umbilical artery, reducing resistance and increasing blood flow.

**Why Each Wrong Option is Incorrect**
**Option A:** An increase in S/D ratio with increasing gestational age is typically associated with fetal distress, placental insufficiency, and increased placental resistance, not a normal adaptation to increasing gestational age.
**Option B:** A constant S/D ratio with increasing gestational age would suggest a lack of change in placental resistance and fetal blood flow, which is not typically observed in a healthy fetus.
**Option C:** A decrease in S/D ratio with increasing gestational age is a normal adaptation, not an increase.

**Clinical Pearl / High-Yield Fact**
A normal S/D ratio at term is typically < 3.0, and an increase in S/D ratio is associated with an increased risk of fetal distress and perinatal morbidity. **Correct Answer:** D. Decrease