**Core Concept**
The blood-placental barrier is a selective permeability barrier between the maternal and fetal blood supplies, regulating the exchange of nutrients, waste, and drugs between the mother and the fetus. Certain drugs can cross this barrier, potentially affecting fetal development and well-being.

**Why the Correct Answer is Right**
A drug's ability to cross the blood-placental barrier depends on its molecular weight, lipid solubility, and degree of ionization. Lipid-soluble, non-ionized molecules can easily diffuse across the placental barrier, whereas polar, ionized molecules are generally excluded. Some drugs, such as antibiotics and anticonvulsants, can cross the placenta in significant amounts, posing risks to the fetus. However, certain drugs, like heparin, are large, negatively charged molecules that are unable to cross the blood-placental barrier.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as this drug is known to cross the blood-placental barrier and can have teratogenic effects on the fetus.
**Option B:** Incorrect, as this drug is lipophilic and can easily diffuse across the placental barrier, potentially causing fetal harm.
**Option C:** Incorrect, as this drug is a polar molecule that can cross the blood-placental barrier, albeit to a lesser extent than lipid-soluble drugs.

**Clinical Pearl / High-Yield Fact**
When prescribing medications to pregnant women, it's essential to consider the potential risks to the fetus and choose drugs that are less likely to cross the blood-placental barrier. Heparin is often used as an alternative to warfarin in pregnant women with thromboembolic disorders.

**Correct Answer:** D.