4. How do chemicals affect the foetus?

The placental barrier in women consists of a triple layer of cells through which substances present in the mother's blood must pass to reach the fetus. It is relatively impermeable to high-molecular-weight chemicals (< 1,000 daltons). It does, however, allow small substances (nanomaterials) with low molecular weight (MW) to pass through. Most drugs have a PM of less than 500 daltons.

The most rapid transplacental passage occurs for lipophilic, non-ionized substances at physiological pH (pH 7). Water-soluble substances pass through hydrophilic capillaries (aquaporins). Their passage depends on molecule size (PM less than 800 daltons).

The human placenta appears to contain an enzymatic system that biotransforms xenobiotics.