## Core Concept
The blood-placental barrier, also known as the placental barrier, is a selectively permeable barrier that separates the maternal and fetal circulations. It allows certain substances to pass through while restricting others, playing a critical role in fetal development and protection.

## Why the Correct Answer is Right
Heparin is a large, negatively charged polysaccharide molecule that does not cross the placental barrier due to its size and charge. This characteristic makes it a preferred anticoagulant during pregnancy, as it does not affect the fetus.

## Why Each Wrong Option is Incorrect
* **Option A:** Warfarin, being a small, lipophilic molecule, can cross the placental barrier. Its use during pregnancy is associated with teratogenic effects and fetal bleeding.
* **Option B:** Insulin, though a large protein, can be transported across the placenta through specific transport mechanisms, but its primary concern is related to maternal glucose control affecting the fetus indirectly.
* **Option C:** Thyroxine (T4) and triiodothyronine (T3) can cross the placenta, and maternal thyroid hormones play a crucial role in fetal development.

## Clinical Pearl / High-Yield Fact
A key clinical consideration is that drugs which are highly protein-bound or have a high molecular weight are less likely to cross the placental barrier. Heparin, being a large molecule, exemplifies this principle and is used clinically for its safety profile in pregnancy.

## Correct Answer: D. Heparin.